Loopback and audio only mode.

Adds a loopback button that will connect to itself by simulating another client connection to the web socket server.

Adds an audio only mode switch.

BUG=

Review URL: https://codereview.webrtc.org/1334003002

Cr-Commit-Position: refs/heads/master@{#10153}

webrtc/examples/objc/AppRTCDemo/mac/APPRTCViewController.m

@@ -280,7 +280,7 @@ static NSUInteger const kLogViewHeight = 280;
         didEnterRoomId:(NSString*)roomId {
   [_client disconnect];
   ARDAppClient *client = [[ARDAppClient alloc] initWithDelegate:self];
-  [client connectToRoomWithId:roomId options:nil];
+  [client connectToRoomWithId:roomId isLoopback:NO isAudioOnly:NO];
   _client = client;
 }