Add PeerConnectionObserver::OnRemoveTrack callback.

webrtc/api/peerconnectioninterface.h

Index: webrtc/api/peerconnectioninterface.h
diff --git a/webrtc/api/peerconnectioninterface.h b/webrtc/api/peerconnectioninterface.h
index 65c1278320b1c4e8a7265e1d91edc7f1ea4e57f0..0eb2c6efb0fb9244324625e94f712e0ac12bba1e 100644
--- a/webrtc/api/peerconnectioninterface.h
+++ b/webrtc/api/peerconnectioninterface.h
@@ -861,6 +861,10 @@ class PeerConnectionObserver {
       rtc::scoped_refptr<RtpReceiverInterface> receiver,
       const std::vector<rtc::scoped_refptr<MediaStreamInterface>>& streams) {}
 
+  virtual void OnRemoveTrack(
+    rtc::scoped_refptr<RtpReceiverInterface> receiver,
+    const std::vector<rtc::scoped_refptr<MediaStreamInterface>>& streams) {}
+
  protected:
   // Dtor protected as objects shouldn't be deleted via this interface.
   ~PeerConnectionObserver() {}