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 ccca18af67d9074a03b700a965d730a5615f1909..fc362de4957d682a86af3df200f378f16451c33d 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;
 }
 
@@ -1956,6 +1958,11 @@ 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(),