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/ios/ARDVideoCallViewController.h

@@ -12,6 +12,8 @@
 
 @interface ARDVideoCallViewController : UIViewController
 
-- (instancetype)initForRoom:(NSString *)room;
+- (instancetype)initForRoom:(NSString *)room
+                 isLoopback:(BOOL)isLoopback
+                isAudioOnly:(BOOL)isAudioOnly;
 
 @end