message: VP9 decoder: replace DCHECK with error message
  Cr-Commit-Position: refs/heads/master@{#29567}
diff: @@ -1746,7 +1746,8 @@ int VP9DecoderImpl::ReturnFrame(
  img->stride[VPX_PLANE_V] / 2, rtc::KeepRefUntilDone(img_buffer));
  break;
  default:
  - RTC_NOTREACHED();
  + RTC_LOG(LS_ERROR) << "Unsupported bit depth produced by the decoder: "
  + << img->bit_depth;
  return WEBRTC_VIDEO_CODEC_NO_OUTPUT;
  }
  