Add runtime check for metal support.

Bug: None
Change-Id: I1bc64c072c3ccf3430e084f9598931c159e6b039
Reviewed-on: https://webrtc-review.googlesource.com/70800
Reviewed-by: Peter Hanspers <peterhanspers@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23072}

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

@@ -64,7 +64,7 @@
 
 + (BOOL)isMetalAvailable {
 #if defined(RTC_SUPPORTS_METAL)
-  return YES;
+  return MTLCreateSystemDefaultDevice() != nil;
 #else
   return NO;
 #endif