Added a callback function OnAddTrack to PeerConnectionObserver

webrtc/api/peerconnectioninterface.h

Index: webrtc/api/peerconnectioninterface.h
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index ea056bfc98fa2a39a1935a911988e3a1be35d897..f05e148c1cd3ec20ab3f712c736e79ec48a29866 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -593,6 +593,11 @@ class PeerConnectionObserver {
   // Called when the ICE connection receiving status changes.
   virtual void OnIceConnectionReceivingChange(bool receiving) {}
 
+  // Called when a track is added to streams.
+  virtual void OnAddTrack(
+      rtc::scoped_refptr<RtpReceiverInterface> receiver,
+      std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams) = 0;

[Taylor Brandstetter, 2016/11/17 00:34:23]

Since Chromium has PeerConnectionObserver subclasses, this will need to have a
default implementation too in order to not break Chromium. Just like the above
two methods with the "TODO(honghaiz)" comment.

[Zhi Huang, 2016/11/17 01:42:06]

Ah, I see. I thought that "TODO" is something we can do now.

+
  protected:
   // Dtor protected as objects shouldn't be deleted via this interface.
   ~PeerConnectionObserver() {}