Adding "first packet received" notification to PeerConnectionObserver.

talk/app/webrtc/peerconnectioninterface.h

Index: talk/app/webrtc/peerconnectioninterface.h
diff --git a/talk/app/webrtc/peerconnectioninterface.h b/talk/app/webrtc/peerconnectioninterface.h
index b9afbad204ce354a7b3294e1fc9409cae615739b..4df729a28c4096bb5abfdff985dcc73cb624dd3f 100644
--- a/talk/app/webrtc/peerconnectioninterface.h
+++ b/talk/app/webrtc/peerconnectioninterface.h
@@ -484,6 +484,9 @@ class PeerConnectionObserver {
   // Called when the ICE connection receiving status changes.
   virtual void OnIceConnectionReceivingChange(bool receiving) {}
 
+  // Called when the first RTP or SCTP packet is received.
+  virtual void OnFirstPacketReceived() {}
+
  protected:
   // Dtor protected as objects shouldn't be deleted via this interface.
   ~PeerConnectionObserver() {}