Implement ObjC DataChannel wrapper

R=fischman@webrtc.org
BUG=3112

Review URL: https://webrtc-codereview.appspot.com/16369004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6031 4adac7df-926f-26a2-2b94-8c16560cd09d

talk/examples/ios/AppRTCDemo/APPRTCAppDelegate.m

@@ -150,6 +150,11 @@
   });
 }
 
+- (void)peerConnection:(RTCPeerConnection*)peerConnection
+    didOpenDataChannel:(RTCDataChannel*)dataChannel {
+  NSAssert(NO, @"AppRTC doesn't use DataChannels");
+}
+
 #pragma mark - Private
 
 - (void)displayLogMessage:(NSString*)message {