Add Metal video view in AppRTCMobile and metal availability macro.

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLVideoView.m

Index: webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLVideoView.m
diff --git a/webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLVideoView.m b/webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLVideoView.m
index 2a18736b25071bba0b2636b9b2bc975295e47dc3..5b85c8a1360f0d03e2dcf4bc83cdcb158a494e16 100644
--- a/webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLVideoView.m
+++ b/webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLVideoView.m
@@ -51,9 +51,9 @@
 #pragma mark - Private
 
 + (BOOL)isMetalAvailable {
-#if defined(__OBJC__) && COREVIDEO_SUPPORTS_METAL
+#if RTC_SUPPORTS_METAL

[tkchin_webrtc, 2017/02/28 18:47:33]

Do we need to expose the define in the header if it's returned in a function
like this? (Or should we just remove this function?)
Also, why are the two defines different?

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

The previous define I realized is not correct. COREVIDEO_SUPPORTS_METAL checks
only if TARGET_OS_IPHONE.
That's not enough because we need to check for cpu architecture as well (this
check will fail for armv7 for instance).
More info in this radar report http://openradar.appspot.com/18498494.
The isMetalAvailable is just convenience to not have #ifdef checks through the
class

[tkchin_webrtc, 2017/03/01 19:16:38]

Gotcha. Ok, if we're sure that Metal support is known at compile time vs runtime
then I'm good with exposing the define publicly. (but please check that that is
true - e.g. iOS10SDK on older device like iPhone 5C)

   return YES;
-#elif
+#else
   return NO;
 #endif
 }