media/gpu/v4l2vda: add missing thread DCHECK

CreateOutputBuffers() was not checking that it was running in the right
thread. Fix this.

BUG=None
TEST=VDAtest passes on Hana.

Change-Id: Ia159b48ec9ca0a0f51049ff1753bdd213722ab29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2026837
Commit-Queue: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#736319}

media/gpu/v4l2/v4l2_video_decode_accelerator.cc

@@ -2463,6 +2463,7 @@ bool V4L2VideoDecodeAccelerator::ProcessFrame(int32_t bitstream_buffer_id,
 
 bool V4L2VideoDecodeAccelerator::CreateOutputBuffers() {
   VLOGF(2);
+  DCHECK(decoder_thread_.task_runner()->BelongsToCurrentThread());
   DCHECK(decoder_state_ == kInitialized ||
          decoder_state_ == kChangingResolution);
   DCHECK(output_queue_);