Reland of Add Metal video view in AppRTCMobile and metal availability macro. (patchset #1 id:1 of https://codereview.webrtc.org/2739793003/ )

Reason for revert:
Fixing rendering issues in AppRTCMobile

Original issue's description:
> Revert of Add Metal video view in AppRTCMobile and metal availability macro. (patchset #5 id:80001 of https://codereview.webrtc.org/2722583002/ )
>
> Reason for revert:
> Breaks AppRTCMobile
>
> Original issue's description:
> > 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}
> > Committed: 154a7bb877
>
> TBR=magjed@webrtc.org,tkchin@webrtc.org,denicija@webrtc.org
> # Not skipping CQ checks because original CL landed more than 1 days ago.
> BUG=webrtc:7079
>
> Review-Url: https://codereview.webrtc.org/2739793003
> Cr-Commit-Position: refs/heads/master@{#17119}
> Committed: cbbd8c76e8

TBR=magjed@webrtc.org,tkchin@webrtc.org,denicija@webrtc.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=webrtc:7079

Review-Url: https://codereview.webrtc.org/2742603003
Cr-Commit-Position: refs/heads/master@{#17139}

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