Add the OnAddTrack callback for Objective-C wrapper.

webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+Private.h

Index: webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+Private.h
diff --git a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+Private.h b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+Private.h
index 60ab868a7c515a9dec6a54f7a4eafe008826cb29..e9fa4975eb56ec64d24cae15f3f94454426d9b00 100644
--- a/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+Private.h
+++ b/webrtc/sdk/objc/Framework/Classes/RTCPeerConnection+Private.h
@@ -49,8 +49,17 @@ class PeerConnectionDelegateAdapter : public PeerConnectionObserver {
   void OnIceCandidatesRemoved(
       const std::vector<cricket::Candidate>& candidates) override;
 
+  void OnAddTrack(

[tkchin_webrtc, 2016/11/22 20:41:34]

what is the distinction between track and receiver? Strange that we're getting
callback that a track is being added, but instead get a receiver.

[Zhi Huang, 2016/11/22 23:38:42]

There is one track attached to one RtpReceiver.

According to the spec, we need to provide RtpRecceiver, Track, MediaStreams and
Transceivers.

We can get the Track from RtpReceiver and we haven't implemented the
Transceivers in native C++ yet. This is why we have these two parameters in the
call back.

+      rtc::scoped_refptr<RtpReceiverInterface> receiver,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) override;
+
  private:
-  __weak RTCPeerConnection *peer_connection_;
+  RTCMediaStream* GetOrCreateObjcStream(
+      rtc::scoped_refptr<MediaStreamInterface> stream);
+
+  __weak RTCPeerConnection* peer_connection_;
+  NSMutableDictionary* nativeToObjcMediaStream_ =
+      [NSMutableDictionary dictionary];

[tkchin_webrtc, 2016/11/22 20:41:34]

Shouldn't this be initialized in ctor?

[Zhi Huang, 2016/11/22 23:38:42]

Done.

 };
 
 } // namespace webrtc