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/Classes/Metal/RTCMTLVideoView.m

@@ -51,9 +51,9 @@
 #pragma mark - Private
 
 + (BOOL)isMetalAvailable {
-#if defined(__OBJC__) && COREVIDEO_SUPPORTS_METAL
+#if defined(RTC_SUPPORTS_METAL)
   return YES;
-#elif
+#else
   return NO;
 #endif
 }