Add a race-checking mechanism.

webrtc/common_video/incoming_video_stream.cc

Index: webrtc/common_video/incoming_video_stream.cc
diff --git a/webrtc/common_video/incoming_video_stream.cc b/webrtc/common_video/incoming_video_stream.cc
index f5a52fcba335f25d1a1a7b7d53773d519d3aec5f..c6b638686f3ae8ad18f5b2e3fafc208cc30c9e84 100644
--- a/webrtc/common_video/incoming_video_stream.cc
+++ b/webrtc/common_video/incoming_video_stream.cc
@@ -49,9 +49,7 @@ IncomingVideoStream::~IncomingVideoStream() {
 }
 
 void IncomingVideoStream::OnFrame(const VideoFrame& video_frame) {
-  // Most of the time we'll be on a decoder thread here, but when using
-  // VideoToolbox on iOS, we'll get called on a thread from a thread pool.
-
+  RTC_CHECK_RUNS_SINGLE_THREADED(&decoder_race_checker_);

[pbos-webrtc, 2016/06/27 14:07:59]

This is a CHECK (not a DCHECK) because we have many decoder implementations and
a lot of them may have significantly lower debug coverage.

   // Hand over or insert frame.
   rtc::CritScope csB(&buffer_critsect_);
   if (render_buffers_->AddFrame(video_frame) == 1) {