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/ARDRoomServerClient.h

@@ -17,6 +17,7 @@
 @protocol ARDRoomServerClient <NSObject>
 
 - (void)joinRoomWithRoomId:(NSString *)roomId
+                isLoopback:(BOOL)isLoopback
          completionHandler:(void (^)(ARDJoinResponse *response,
                                      NSError *error))completionHandler;