Exposing RtpSenders and RtpReceivers from PeerConnection.

talk/app/webrtc/mediastreamsignaling.h

Index: talk/app/webrtc/mediastreamsignaling.h
diff --git a/talk/app/webrtc/mediastreamsignaling.h b/talk/app/webrtc/mediastreamsignaling.h
index 87eede6305d26ad1956b74a7f3808e430b1e800b..18c24b7e1d9bac70ed1cd221cfa94bf92f4ff2f6 100644
--- a/talk/app/webrtc/mediastreamsignaling.h
+++ b/talk/app/webrtc/mediastreamsignaling.h
@@ -66,12 +66,14 @@ class MediaStreamSignalingObserver {
   // Triggered when the remote SessionDescription has a new audio track.
   virtual void OnAddRemoteAudioTrack(MediaStreamInterface* stream,
                                      AudioTrackInterface* audio_track,
-                                     uint32 ssrc) = 0;
+                                     uint32 ssrc,
+                                     const std::string& mid) = 0;

[pthatcher1, 2015/09/17 04:25:46]

I don't see how MID is even useful here.  I think we should just remove it from
RtpSenders and RtpReceivers for now.  

Here's an idea:  instead of getting the (stream, track, ssrc, mid) from the SDP,
why not instead get (sender_id, ssrc, track), where the sender_id is MSID for
"Plan B" SDP and MID for Unified Plan later?   

We should also add a TODO to change all calls through BaseChannel/MediaChannel
to use "sender_id" instead of SSRC.   Then it would just be (sender_id, track).

We should also add a TODO to change addTrack to put the track on the RtpSender
before the SDP comes in.  Then this callback could just be (sender_id).

[Taylor Brandstetter, 2015/09/23 00:10:45]

It's not useful right now. It's just something I knew we'd need eventually, so I
thought I'd go ahead and add it. But if it's just going to be clutter, I'll go
ahead and remove it.

Also, I'll do the "sender_id" change in the next CL, since it's more relevant
there.

 
   // Triggered when the remote SessionDescription has a new video track.
   virtual void OnAddRemoteVideoTrack(MediaStreamInterface* stream,
                                      VideoTrackInterface* video_track,
-                                     uint32 ssrc) = 0;
+                                     uint32 ssrc,
+                                     const std::string& mid) = 0;
 
   // Triggered when the remote SessionDescription has removed an audio track.
   virtual void OnRemoveRemoteAudioTrack(MediaStreamInterface* stream,
@@ -84,12 +86,14 @@ class MediaStreamSignalingObserver {
   // Triggered when the local SessionDescription has a new audio track.
   virtual void OnAddLocalAudioTrack(MediaStreamInterface* stream,
                                     AudioTrackInterface* audio_track,
-                                    uint32 ssrc) = 0;
+                                    uint32 ssrc,
+                                    const std::string& mid) = 0;
 
   // Triggered when the local SessionDescription has a new video track.
   virtual void OnAddLocalVideoTrack(MediaStreamInterface* stream,
                                     VideoTrackInterface* video_track,
-                                    uint32 ssrc) = 0;
+                                    uint32 ssrc,
+                                    const std::string& mid) = 0;
 
   // Triggered when the local SessionDescription has removed an audio track.
   virtual void OnRemoveLocalAudioTrack(MediaStreamInterface* stream,
@@ -276,6 +280,10 @@ class MediaStreamSignaling : public sigslot::has_slots<> {
     // sent but no MSID is given.
     bool default_video_track_needed;
 
+    // mid strings associated with the empty audio/video content descriptions.
+    std::string default_audio_mid;
+    std::string default_video_mid;
+
     bool IsDefaultMediaStreamNeeded() {
       return !msid_supported && (default_audio_track_needed ||
           default_video_track_needed);
@@ -286,14 +294,16 @@ class MediaStreamSignaling : public sigslot::has_slots<> {
     TrackInfo() : ssrc(0) {}
     TrackInfo(const std::string& stream_label,
               const std::string track_id,
-              uint32 ssrc)
+              uint32 ssrc,
+              const std::string& mid)
         : stream_label(stream_label),
           track_id(track_id),
-          ssrc(ssrc) {
-    }
+          ssrc(ssrc),
+          mid(mid) {}
     std::string stream_label;
     std::string track_id;
     uint32 ssrc;
+    std::string mid;
   };
   typedef std::vector<TrackInfo> TrackInfos;
 
@@ -304,6 +314,7 @@ class MediaStreamSignaling : public sigslot::has_slots<> {
   void UpdateRemoteStreamsList(
       const std::vector<cricket::StreamParams>& streams,
       cricket::MediaType media_type,
+      const std::string& mid,
       StreamCollection* new_streams);
 
   // Triggered when a remote track has been seen for the first time in a remote
@@ -313,7 +324,8 @@ class MediaStreamSignaling : public sigslot::has_slots<> {
   void OnRemoteTrackSeen(const std::string& stream_label,
                          const std::string& track_id,
                          uint32 ssrc,
-                         cricket::MediaType media_type);
+                         cricket::MediaType media_type,
+                         const std::string& mid);
 
   // Triggered when a remote track has been removed from a remote session
   // description. It removes the remote track with id |track_id| from a remote
@@ -343,7 +355,8 @@ class MediaStreamSignaling : public sigslot::has_slots<> {
   // For each new or removed StreamParam NotifyLocalTrackAdded or
   // NotifyLocalTrackRemoved in invoked.
   void UpdateLocalTracks(const std::vector<cricket::StreamParams>& streams,
-                         cricket::MediaType media_type);
+                         cricket::MediaType media_type,
+                         const std::string& mid);
 
   // Triggered when a local track has been seen for the first time in a local
   // session description.
@@ -354,7 +367,8 @@ class MediaStreamSignaling : public sigslot::has_slots<> {
   void OnLocalTrackSeen(const std::string& stream_label,
                         const std::string& track_id,
                         uint32 ssrc,
-                        cricket::MediaType media_type);
+                        cricket::MediaType media_type,
+                        const std::string& mid);
 
   // Triggered when a local track has been removed from a local session
   // description.