Adding "first packet received" notification to PeerConnectionObserver.

webrtc/examples/objc/AppRTCDemo/ARDAppClient.m

 /*
  *  Copyright 2014 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 366 matching lines @@
     ARDICECandidateMessage *message =
         [[ARDICECandidateMessage alloc] initWithCandidate:candidate];
     [self sendSignalingMessage:message];
   });
 }
 
 - (void)peerConnection:(RTCPeerConnection *)peerConnection
     didOpenDataChannel:(RTCDataChannel *)dataChannel {
 }
 
+- (void)peerConnectionOnFirstMediaPacketReceived:
+    (RTCPeerConnection *)peerConnection {
+  RTCLog(@"Received first media packet.");
+}
+
 #pragma mark - RTCStatsDelegate
 
 - (void)peerConnection:(RTCPeerConnection *)peerConnection
            didGetStats:(NSArray *)stats {
   dispatch_async(dispatch_get_main_queue(), ^{
     [_delegate appClient:self didGetStats:stats];
   });
 }
 
 #pragma mark - RTCSessionDescriptionDelegate
@@ skipping 327 matching lines @@
                                          code:kARDAppClientErrorInvalidRoom
                                      userInfo:@{
         NSLocalizedDescriptionKey: @"Invalid room.",
       }];
       break;
   }
   return error;
 }
 
 @end