Move setting videoFrameSize into the main dispatch queue.

Bug: webrtc:9179
Change-Id: I46b19b67c267013d600dc754ba2bcf1ca9c038e6
Reviewed-on: https://webrtc-review.googlesource.com/71996
Reviewed-by: Zeke Chin <tkchin@webrtc.org>
Reviewed-by: Kári Helgason <kthelgason@webrtc.org>
Commit-Queue: JT Teh <jtteh@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23007}

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

@@ -157,8 +157,8 @@
 
 - (void)setSize:(CGSize)size {
   self.metalView.drawableSize = size;
-  _videoFrameSize = size;
   dispatch_async(dispatch_get_main_queue(), ^{
+    _videoFrameSize = size;
     [self.delegate videoView:self didChangeVideoSize:size];
   });
 }