[media] Allow RTCVDA::Release() on DecodingThread.

RTCVideoDecoderAdapter::Release() is called on the DecoderThread when
InitDecode() fails.

Bug: 934798
Change-Id: I8157d73511db0639d2013e2f4d62293a47ceaf5c
Reviewed-on: https://chromium-review.googlesource.com/c/1483683Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Commit-Queue: Dan Sanders <sandersd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634851}

content/renderer/media/webrtc/rtc_video_decoder_adapter.cc

@@ -287,7 +287,6 @@ int32_t RTCVideoDecoderAdapter::RegisterDecodeCompleteCallback(
 
 int32_t RTCVideoDecoderAdapter::Release() {
   DVLOG(1) << __func__;
-  DCHECK_CALLED_ON_VALID_THREAD(worker_thread_checker_);
 
   base::AutoLock auto_lock(lock_);
   pending_buffers_.clear();

content/renderer/media/webrtc/rtc_video_decoder_adapter.h

@@ -72,7 +72,7 @@ class CONTENT_EXPORT RTCVideoDecoderAdapter : public webrtc::VideoDecoder {
                  bool missing_frames,
                  const webrtc::CodecSpecificInfo* codec_specific_info,
                  int64_t render_time_ms) override;
-  // Called on the worker thread.
+  // Called on the worker thread and on the DecodingThread.
   int32_t Release() override;
   // Called on the worker thread and on the DecodingThread.
   const char* ImplementationName() const override;