Adding "first packet received" notification to PeerConnectionObserver.

talk/app/webrtc/objc/RTCPeerConnectionObserver.h

 /*
  * libjingle
  * Copyright 2013 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are met:
  *
  *  1. Redistributions of source code must retain the above copyright notice,
  *     this list of conditions and the following disclaimer.
  *  2. Redistributions in binary form must reproduce the above copyright notice,
@@ skipping 50 matching lines @@
   void OnIceConnectionChange(
       PeerConnectionInterface::IceConnectionState new_state) override;
 
   // Called any time the ICEGatheringState changes
   void OnIceGatheringChange(
       PeerConnectionInterface::IceGatheringState new_state) override;
 
   // New Ice candidate have been found.
   void OnIceCandidate(const IceCandidateInterface* candidate) override;
 
+  // Called when the first RTP packet is received.
+  void OnFirstMediaPacketReceived() override;
+
  private:
   __weak RTCPeerConnection* _peerConnection;
 };
 
 } // namespace webrtc