Adding "first packet received" notification to PeerConnectionObserver.

R=glaznev@webrtc.org, pthatcher@webrtc.org, tkchin@webrtc.org

Committed: https://crrev.com/42265a8cc3b3f3db4aa2c29005aed2fb4393adef
Cr-Commit-Position: refs/heads/master@{#11401}

Review URL: https://codereview.webrtc.org/1581693006 .

Cr-Commit-Position: refs/heads/master@{#11404}

webrtc/examples/objc/AppRTCDemo/ARDAppClient.m

@@ -384,6 +384,11 @@ static NSInteger const kARDAppClientErrorInvalidRoom = -6;
     didOpenDataChannel:(RTCDataChannel *)dataChannel {
 }
 
+- (void)peerConnectionOnFirstMediaPacketReceived:
+    (RTCPeerConnection *)peerConnection {
+  RTCLog(@"Received first media packet.");
+}
+
 #pragma mark - RTCStatsDelegate
 
 - (void)peerConnection:(RTCPeerConnection *)peerConnection