Add PeerConnectionObserver::OnRemoveTrack callback.

webrtc/pc/peerconnection.cc

Index: webrtc/pc/peerconnection.cc
diff --git a/webrtc/pc/peerconnection.cc b/webrtc/pc/peerconnection.cc
index f006b23cbda5197d9646f9e233ab15176f99b3af..240ef92e77530d0a86c3a6cf8883dcf09dcda001 100644
--- a/webrtc/pc/peerconnection.cc
+++ b/webrtc/pc/peerconnection.cc
@@ -1486,14 +1486,21 @@ void PeerConnection::CreateVideoReceiver(MediaStreamInterface* stream,
 
 // TODO(deadbeef): Keep RtpReceivers around even if track goes away in remote
 // description.
-void PeerConnection::DestroyReceiver(const std::string& track_id) {
+void PeerConnection::DestroyReceiver(MediaStreamInterface* stream,
+                                     const std::string& track_id) {
   auto it = FindReceiverForTrack(track_id);
   if (it == receivers_.end()) {
     LOG(LS_WARNING) << "RtpReceiver for track with id " << track_id
                     << " doesn't exist.";
   } else {
+    rtc::scoped_refptr<RtpReceiverInterface> receiver(*it);
+    std::vector<rtc::scoped_refptr<MediaStreamInterface>> streams;
+    streams.push_back(rtc::scoped_refptr<MediaStreamInterface>(stream));
+
     (*it)->internal()->Stop();
     receivers_.erase(it);
+
+    observer_->OnRemoveTrack(receiver, streams);
   }
 }
 
@@ -1917,7 +1924,7 @@ void PeerConnection::OnRemoteTrackRemoved(const std::string& stream_label,
   if (media_type == cricket::MEDIA_TYPE_AUDIO) {
     // When the MediaEngine audio channel is destroyed, the RemoteAudioSource
     // will be notified which will end the AudioRtpReceiver::track().
-    DestroyReceiver(track_id);
+    DestroyReceiver(stream, track_id);
     rtc::scoped_refptr<AudioTrackInterface> audio_track =
         stream->FindAudioTrack(track_id);
     if (audio_track) {
@@ -1926,7 +1933,7 @@ void PeerConnection::OnRemoteTrackRemoved(const std::string& stream_label,
   } else if (media_type == cricket::MEDIA_TYPE_VIDEO) {
     // Stopping or destroying a VideoRtpReceiver will end the
     // VideoRtpReceiver::track().
-    DestroyReceiver(track_id);
+    DestroyReceiver(stream, track_id);
     rtc::scoped_refptr<VideoTrackInterface> video_track =
         stream->FindVideoTrack(track_id);
     if (video_track) {