Add Metal video view in AppRTCMobile and metal availability macro.

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

 /*
  *  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 "ARDVideoCallView.h"
 
 #import <AVFoundation/AVFoundation.h>
+
+#import <WebRTC/RTCEAGLVideoView.h>
+#import <WebRTC/RTCMTLVideoView.h>
+
 #import "UIImage+ARDUtilities.h"
 
 static CGFloat const kButtonPadding = 16;
 static CGFloat const kButtonSize = 48;
 static CGFloat const kLocalVideoViewSize = 120;
 static CGFloat const kLocalVideoViewPadding = 8;
 static CGFloat const kStatusBarHeight = 20;
 
 @interface ARDVideoCallView () <RTCEAGLVideoViewDelegate>
 @end
 
 @implementation ARDVideoCallView {
   UIButton *_routeChangeButton;
   UIButton *_cameraSwitchButton;
   UIButton *_hangupButton;
   CGSize _remoteVideoSize;
   BOOL _useRearCamera;
 }
 
 @synthesize statusLabel = _statusLabel;
 @synthesize localVideoView = _localVideoView;
 @synthesize remoteVideoView = _remoteVideoView;
 @synthesize statsView = _statsView;
 @synthesize delegate = _delegate;
 
 - (instancetype)initWithFrame:(CGRect)frame {
   if (self = [super initWithFrame:frame]) {
-    _remoteVideoView = [[RTCEAGLVideoView alloc] initWithFrame:CGRectZero];
-    _remoteVideoView.delegate = self;
+
+#if defined(RTC_SUPPORTS_METAL)
+    _remoteVideoView = [[RTCMTLVideoView alloc] initWithFrame:CGRectZero];
+#else
+    RTCEAGLVideoView *remoteView = [[RTCEAGLVideoView alloc] initWithFrame:CGRectZero];
+    remoteView.delegate = self;
+    _remoteVideoView = remoteView;
+#endif
+
     [self addSubview:_remoteVideoView];
 
     _localVideoView = [[RTCCameraPreviewView alloc] initWithFrame:CGRectZero];
     [self addSubview:_localVideoView];
 
     _statsView = [[ARDStatsView alloc] initWithFrame:CGRectZero];
     _statsView.hidden = YES;
     [self addSubview:_statsView];
 
     _routeChangeButton = [UIButton buttonWithType:UIButtonTypeCustom];
@@ skipping 131 matching lines @@
 
 - (void)onHangup:(id)sender {
   [_delegate videoCallViewDidHangup:self];
 }
 
 - (void)didTripleTap:(UITapGestureRecognizer *)recognizer {
   [_delegate videoCallViewDidEnableStats:self];
 }
 
 @end