Add Metal video view in AppRTCMobile and Metal availability macro.

- The RTC_SUPPORTS_METAL macro allows consumers to gracefully handle compilation for different archs that are not supporting Metal.

BUG=webrtc:7079

Review-Url: https://codereview.webrtc.org/2722583002
Cr-Commit-Position: refs/heads/master@{#17004}

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(__aarch64__)
+#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