Add Metal video view in AppRTCMobile and metal availability macro.

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

Index: webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.h
diff --git a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.h b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.h
index 681c457516fdf040770904207659cb55be46ba0d..3c8bdeec4d0937fd95ec9a29b572b7b60d14163a 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.h
+++ b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.h
@@ -12,8 +12,13 @@
 
 #import "WebRTC/RTCVideoRenderer.h"
 
+// Check if metal is supported in WebRTC.
+// NOTE: Currently __arm64__ == Metal.
+#if defined(__arm64__) && __arm64__

[kthelgason, 2017/03/03 12:38:39]

nit: just defined(__arm64__) should be enough. Also, chromium uses __aarch64__
instead, because that's also defined when using gcc. Maybe we should use that
just for the sake of staying consistent.

+#define RTC_SUPPORTS_METAL
+#endif
+
 NS_ASSUME_NONNULL_BEGIN
-RTC_EXPORT
 
 /**
  * RTCMTLVideoView is thin wrapper around MTKView.
@@ -22,6 +27,8 @@ RTC_EXPORT
  * bounds using Metal.
  */
 NS_CLASS_AVAILABLE_IOS(9)
+
+RTC_EXPORT
 @interface RTCMTLVideoView : UIView <RTCVideoRenderer>
 
 @end