Add Metal video view in AppRTCMobile and metal availability macro.

webrtc/examples/objc/AppRTCMobile/ios/ARDVideoCallView.h

 /*
  *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #import <UIKit/UIKit.h>
 
-#import "WebRTC/RTCCameraPreviewView.h"
-#import "WebRTC/RTCEAGLVideoView.h"
+#import <WebRTC/RTCCameraPreviewView.h>
+#import <WebRTC/RTCVideoRenderer.h>
 
 #import "ARDStatsView.h"
 
 @class ARDVideoCallView;
 @protocol ARDVideoCallViewDelegate <NSObject>
 
 // Called when the camera switch button is pressed.
 - (void)videoCallViewDidSwitchCamera:(ARDVideoCallView *)view;
 
 // Called when the route change button is pressed.
 - (void)videoCallViewDidChangeRoute:(ARDVideoCallView *)view;
 
 // Called when the hangup button is pressed.
 - (void)videoCallViewDidHangup:(ARDVideoCallView *)view;
 
 // Called when stats are enabled by triple tapping.
 - (void)videoCallViewDidEnableStats:(ARDVideoCallView *)view;
 
 @end
 
 // Video call view that shows local and remote video, provides a label to
 // display status, and also a hangup button.
 @interface ARDVideoCallView : UIView
 
 @property(nonatomic, readonly) UILabel *statusLabel;
 @property(nonatomic, readonly) RTCCameraPreviewView *localVideoView;
-@property(nonatomic, readonly) RTCEAGLVideoView *remoteVideoView;
+@property(nonatomic, readonly) __kindof UIView<RTCVideoRenderer> *remoteVideoView;
 @property(nonatomic, readonly) ARDStatsView *statsView;
 @property(nonatomic, weak) id<ARDVideoCallViewDelegate> delegate;
 
 @end