Add Metal video view in AppRTCMobile and metal availability macro.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.h

 /*
  *  Copyright 2017 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 <Foundation/Foundation.h>
 
 #import "WebRTC/RTCVideoRenderer.h"
 
+// Check if metal is supported in WebRTC.
+// NOTE: Currently __arm64__ == Metal.
+#if defined(__arm64__) && __arm64__
+  #define RTC_SUPPORTS_METAL 1

[magjed_webrtc, 2017/02/28 17:28:15]

Don't indent and just do:
#if defined(__arm64__) && __arm64__
#define RTC_SUPPORTS_METAL
#endif
and then:
#if defined(RTC_SUPPORTS_METAL)

[daniela-webrtc, 2017/03/01 13:40:22]

Done.

+#else
+  #define RTC_SUPPORTS_METAL 0
+#endif
+
 NS_ASSUME_NONNULL_BEGIN
-RTC_EXPORT
 
 /**
  * RTCMTLVideoView is thin wrapper around MTKView.
  *
  * It has id<RTCVideoRenderer> property that renders video frames in the view's
  * bounds using Metal.
  */
 NS_CLASS_AVAILABLE_IOS(9)
+
+RTC_EXPORT
 @interface RTCMTLVideoView : UIView <RTCVideoRenderer>
 
 @end
 NS_ASSUME_NONNULL_END