Fix bug in reporting decode errors in VideoToolbox

The reference back to the decoder class in the decode callback
was null. Due to the amazing properties of ObjC this led to the
setError call to silently fail.

Bug: webrtc:8600
Change-Id: I3f70fbe4c9d533c8612d0bc7bc40813252e492fd
Reviewed-on: https://webrtc-review.googlesource.com/52460
Reviewed-by: Peter Hanspers <peterhanspers@webrtc.org>
Commit-Queue: Kári Helgason <kthelgason@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22021}

sdk/objc/Framework/Classes/VideoToolbox/RTCVideoDecoderH264.mm

@@ -236,7 +236,7 @@ void decompressionOutputCallback(void *decoderRef,
     pixelFormat = nullptr;
   }
   VTDecompressionOutputCallbackRecord record = {
-      decompressionOutputCallback, nullptr,
+      decompressionOutputCallback, (__bridge void *)self,
   };
   OSStatus status = VTDecompressionSessionCreate(
       nullptr, _videoFormat, nullptr, attributes, &record, &_decompressionSession);