Updates to VCMDecodedFrameCallback, VideoReceiver and a few related classes/tests.

webrtc/video/video_stream_decoder.cc

Index: webrtc/video/video_stream_decoder.cc
diff --git a/webrtc/video/video_stream_decoder.cc b/webrtc/video/video_stream_decoder.cc
index b0308f25d85fba6bad556bc4d0118fe5d1d026d9..17f29dfe2a88e42faad7124c5c2f2e3486eb7f1e 100644
--- a/webrtc/video/video_stream_decoder.cc
+++ b/webrtc/video/video_stream_decoder.cc
@@ -61,6 +61,10 @@ VideoStreamDecoder::VideoStreamDecoder(
 }
 
 VideoStreamDecoder::~VideoStreamDecoder() {
+  // Note: There's an assumption at this point that the decoder thread is
+  // *not* running. If it was, then there could be a race for each of these
+  // callbacks.
+
   // Unset all the callback pointers that we set in the ctor.
   video_receiver_->RegisterPacketRequestCallback(nullptr);
   video_receiver_->RegisterDecoderTimingCallback(nullptr);