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"
 
+RTC_EXPORT

[kthelgason, 2017/02/28 10:03:10]

This should be right before the exported symbol, in this case the @interface.
Although the NS_CLASS_AVAILABLE macro can come in between.

+
+// Check if metal is supported in WebRTC.
+// NOTE: Currently __arm64__ == Metal.
+#if defined(__arm64__) && __arm64__
+  #define RTC_SUPPORTS_METAL 1
+#else
+  #define RTC_SUPPORTS_METAL 0
+#endif

[kthelgason, 2017/02/28 10:03:10]

I'd want to set this in gn if possible. Are there any drawbacks to that approach
that I'm not thinking through?

[daniela-webrtc, 2017/02/28 14:27:12]

This is intended for the consumers of the API, to be able to determine in
compile time if this class is available for the particular architecture they are
building.
Otherwise they'd get symbol unavailable exceptions or they'd need to do some
sort of runtime creation of the class, like NSClassFromString() to prevent the
compiler from complaining about the missing symbols.

+
 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)
 @interface RTCMTLVideoView : UIView <RTCVideoRenderer>
 
 @end
 NS_ASSUME_NONNULL_END