Support external audio mixer.

webrtc/api/peerconnection.cc

Index: webrtc/api/peerconnection.cc
diff --git a/webrtc/api/peerconnection.cc b/webrtc/api/peerconnection.cc
index 46bdec594a9c90fcd1630db748921237807902d2..e7ec7d4f426308ac6470ae498d6a88a406006850 100644
--- a/webrtc/api/peerconnection.cc
+++ b/webrtc/api/peerconnection.cc
@@ -1353,6 +1353,15 @@ void PeerConnection::Close() {
   session_->Close();
 }
 
+std::vector<uint32_t> PeerConnection::GetRemoteAudioTrackSsrcs() {

[the sun, 2016/12/07 15:56:08]

Is this utility really needed? Can you use PC::remote_streams() and iterate over
streams and call GetAudioTracks() for each stream, fetching the ssrc by calling
id() on each track?

[GeorgeZ, 2016/12/07 18:28:17]

PC::remote_streams() provides rtc::scoped_refptr<StreamCollectionInterface>.
Iterating StreamCollectionInterface can get MediaStreamInterface.
MediaStreamInterface has label() which is not ssrc.
AudioTrackInterface can get from MediaStreamInterface. However
AudioTrackInterface doesn't have ID or Label.

Therefore, I am sure I can get ssrc from PC.

[the sun, 2016/12/08 07:52:29]

AudioTrackInterface inherits from MediaStreamTrackInterface, which has an id()
method. However, I must confess that it's not clear to me whether this is
actually the SSRC or something else. Could you please check if it can be used?
I'd like to avoid adding another method to PeerConnectionInterface if we can
avoid it.

+  std::vector<uint32_t> ssrcs;
+  TrackInfos* infos = GetRemoteTracks(cricket::MEDIA_TYPE_AUDIO);
+  for (const auto& info : *infos)
+    ssrcs.push_back(info.ssrc);
+
+  return ssrcs;
+}
+
 void PeerConnection::OnSessionStateChange(WebRtcSession* /*session*/,
                                           WebRtcSession::State state) {
   switch (state) {