Adding "first packet received" notification to PeerConnectionObserver.

talk/app/webrtc/java/src/org/webrtc/PeerConnection.java

Index: talk/app/webrtc/java/src/org/webrtc/PeerConnection.java
diff --git a/talk/app/webrtc/java/src/org/webrtc/PeerConnection.java b/talk/app/webrtc/java/src/org/webrtc/PeerConnection.java
index 36cd07595cd58a63f01e830eb857d27c4012dacc..92a243f1a72195d54213bfb6f12ed34c8178e161 100644
--- a/talk/app/webrtc/java/src/org/webrtc/PeerConnection.java
+++ b/talk/app/webrtc/java/src/org/webrtc/PeerConnection.java
@@ -86,6 +86,9 @@ public class PeerConnection {
 
     /** Triggered when renegotiation is necessary. */
     public void onRenegotiationNeeded();
+
+    /** Called when the first RTP or SCTP packet is received. */
+    public void onFirstPacketReceived();

[pthatcher1, 2016/01/14 23:50:09]

I'm worried about the "SCTP packet" here.  SCTP may send internal packets before
you even choose to send a message over it, and the BaseChannel doesn't
differentiate.  So this may fire before you're expecting it.

I think we need to update BaseChannel to check for RTP and not just !rtcp here:

https://code.google.com/p/chromium/codesearch#chromium/src/third_party/libjin...

That would remove the chance of being tricked by internal SCTP packets.

Further, I don't think there is any value in firing up when we get the first
"real" SCTP packet, since a small SCTP message will get process as quickly as
one packet anyway, and an application that wants to do signalling over SCTP
would find it impossible to differentiate between the first media and the first
signalling.

[Taylor Brandstetter, 2016/01/15 17:06:58]

I'll just change it to "first media packet received", since like you said,
knowing when the first data message is received is already possible.

   }
 
   /** Java version of PeerConnectionInterface.IceServer. */