Adding "first packet received" notification to PeerConnectionObserver.

talk/app/webrtc/peerconnection.h

Index: talk/app/webrtc/peerconnection.h
diff --git a/talk/app/webrtc/peerconnection.h b/talk/app/webrtc/peerconnection.h
index 03b5853c295258f18d3f5ce33c2b2a816314f927..83c408ba5b028c63ee747c9bb2a871675a0a5514 100644
--- a/talk/app/webrtc/peerconnection.h
+++ b/talk/app/webrtc/peerconnection.h
@@ -326,6 +326,7 @@ class PeerConnection : public PeerConnectionInterface,
   // webrtc::DataChannel should be opened.
   void OnDataChannelOpenMessage(const std::string& label,
                                 const InternalDataChannelInit& config);
+  void OnFirstMediaPacketReceived();
 
   RtpSenderInterface* FindSenderById(const std::string& id);