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..17f307452d70339ff8d5dfb6ddb2cfcd89660bb2 100644
--- a/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.h
+++ b/webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLVideoView.h
@@ -12,9 +12,18 @@
 
 #import "WebRTC/RTCVideoRenderer.h"
 
-NS_ASSUME_NONNULL_BEGIN
 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
+
 /**
  * RTCMTLVideoView is thin wrapper around MTKView.
  *