Adding "first packet received" notification to PeerConnectionObserver.

talk/app/webrtc/webrtcsession.cc

Index: talk/app/webrtc/webrtcsession.cc
diff --git a/talk/app/webrtc/webrtcsession.cc b/talk/app/webrtc/webrtcsession.cc
index c6d1856f0353957224932aeb8e0c076e4ccdfaa9..0270c554fbb1861bc7b7026d92d9e2a67d020fde 100644
--- a/talk/app/webrtc/webrtcsession.cc
+++ b/talk/app/webrtc/webrtcsession.cc
@@ -1844,6 +1844,8 @@ bool WebRtcSession::CreateVoiceChannel(const cricket::ContentInfo* content) {
 
   voice_channel_->SignalDtlsSetupFailure.connect(
       this, &WebRtcSession::OnDtlsSetupFailure);
+  voice_channel_->SignalFirstPacketReceived.connect(
+      this, &WebRtcSession::OnChannelFirstPacketReceived);
 
   SignalVoiceChannelCreated();
   voice_channel_->transport_channel()->SignalSentPacket.connect(
@@ -1861,6 +1863,8 @@ bool WebRtcSession::CreateVideoChannel(const cricket::ContentInfo* content) {
 
   video_channel_->SignalDtlsSetupFailure.connect(
       this, &WebRtcSession::OnDtlsSetupFailure);
+  video_channel_->SignalFirstPacketReceived.connect(
+      this, &WebRtcSession::OnChannelFirstPacketReceived);
 
   SignalVideoChannelCreated();
   video_channel_->transport_channel()->SignalSentPacket.connect(
@@ -1895,6 +1899,14 @@ void WebRtcSession::OnDtlsSetupFailure(cricket::BaseChannel*, bool rtcp) {
            rtcp ? kDtlsSetupFailureRtcp : kDtlsSetupFailureRtp);
 }
 
+void WebRtcSession::OnChannelFirstPacketReceived(cricket::BaseChannel*) {
+  ASSERT(signaling_thread()->IsCurrent());
+  if (!has_received_media_packet_) {
+    has_received_media_packet_ = true;
+    SignalFirstMediaPacketReceived();
+  }
+}
+
 void WebRtcSession::OnDataChannelMessageReceived(
     cricket::DataChannel* channel,
     const cricket::ReceiveDataParams& params,