iOS camera switching video capturer.

Introduces a new capture class derived from cricket::VideoCapturer that
provides the ability to switch cameras and updates AppRTCDemo to use it.
Some future work pending to clean up AppRTCDemo UI.

BUG=4070
R=magjed@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48279005

Cr-Commit-Position: refs/heads/master@{#9137}

talk/examples/objc/AppRTCDemo/ios/ARDVideoCallView.h

@@ -32,6 +32,9 @@
 @class ARDVideoCallView;
 @protocol ARDVideoCallViewDelegate <NSObject>
 
+// Called when the camera switch button is pressed.
+- (void)videoCallViewDidSwitchCamera:(ARDVideoCallView *)view;
+
 // Called when the hangup button is pressed.
 - (void)videoCallViewDidHangup:(ARDVideoCallView *)view;