Adding "first packet received" notification to PeerConnectionObserver.

talk/app/webrtc/peerconnection.cc

Index: talk/app/webrtc/peerconnection.cc
diff --git a/talk/app/webrtc/peerconnection.cc b/talk/app/webrtc/peerconnection.cc
index beae77097836474083ef0afec7955cdf5fe2c52f..d2f8d77b65cbb6bf7dd5b0e11293069080cc5641 100644
--- a/talk/app/webrtc/peerconnection.cc
+++ b/talk/app/webrtc/peerconnection.cc
@@ -663,6 +663,8 @@ bool PeerConnection::Initialize(
       this, &PeerConnection::OnDataChannelDestroyed);
   session_->SignalDataChannelOpenMessage.connect(
       this, &PeerConnection::OnDataChannelOpenMessage);
+  session_->SignalFirstMediaPacketReceived.connect(
+      this, &PeerConnection::OnFirstMediaPacketReceived);
   return true;
 }
 
@@ -2029,6 +2031,11 @@ void PeerConnection::OnDataChannelOpenMessage(
       DataChannelProxy::Create(signaling_thread(), channel));
 }
 
+void PeerConnection::OnFirstMediaPacketReceived() {
+  RTC_DCHECK(signaling_thread()->IsCurrent());
+  observer_->OnFirstMediaPacketReceived();
+}
+
 RtpSenderInterface* PeerConnection::FindSenderById(const std::string& id) {
   auto it =
       std::find_if(senders_.begin(), senders_.end(),