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/ARDMainView.h

@@ -14,7 +14,10 @@
 
 @protocol ARDMainViewDelegate <NSObject>
 
-- (void)mainView:(ARDMainView *)mainView didInputRoom:(NSString *)room;
+- (void)mainView:(ARDMainView *)mainView
+    didInputRoom:(NSString *)room
+      isLoopback:(BOOL)isLoopback
+     isAudioOnly:(BOOL)isAudioOnly;
 
 @end