Adding "first packet received" notification to PeerConnectionObserver.

talk/app/webrtc/webrtcsession.h

Index: talk/app/webrtc/webrtcsession.h
diff --git a/talk/app/webrtc/webrtcsession.h b/talk/app/webrtc/webrtcsession.h
index efb280fc48dc8d632342bf8b5ff648a9d7be11f3..29f9342042925214117a0ec7768f2a924b5bec71 100644
--- a/talk/app/webrtc/webrtcsession.h
+++ b/talk/app/webrtc/webrtcsession.h
@@ -310,6 +310,7 @@ class WebRtcSession : public AudioProviderInterface,
   void OnCertificateReady(
       const rtc::scoped_refptr<rtc::RTCCertificate>& certificate);
   void OnDtlsSetupFailure(cricket::BaseChannel*, bool rtcp);
+  void OnChannelFirstPacketReceived(cricket::BaseChannel*);
 
   // For unit test.
   bool waiting_for_certificate_for_testing() const;
@@ -334,6 +335,9 @@ class WebRtcSession : public AudioProviderInterface,
   sigslot::signal2<const std::string&, const InternalDataChannelInit&>
       SignalDataChannelOpenMessage;
 
+  // Called when the first RTP or SCTP packet is received.
+  sigslot::signal0<> SignalFirstPacketReceived;
+
  private:
   // Indicates the type of SessionDescription in a call to SetLocalDescription
   // and SetRemoteDescription.
@@ -517,6 +521,8 @@ class WebRtcSession : public AudioProviderInterface,
   // Declares the RTCP mux policy for the WebRTCSession.
   PeerConnectionInterface::RtcpMuxPolicy rtcp_mux_policy_;
 
+  bool has_received_packet_ = false;
+
   RTC_DISALLOW_COPY_AND_ASSIGN(WebRtcSession);
 };
 }  // namespace webrtc