message: Use single thread vp9 decoder for fuzzing
  Single thread vp9 decoder is more fuzzer friendly.
  Cr-Commit-Position: refs/heads/master@{#29435}
diff: @@ -1625,10 +1625,20 @@ int VP9DecoderImpl::InitDecode(const VideoCodec* inst, int number_of_cores) {
  vpx_codec_dec_cfg_t cfg;
  memset(&cfg, 0, sizeof(cfg));
  +#ifdef FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
  + // We focus on webrtc fuzzing here, not libvpx itself. Use single thread for
  + // fuzzing, because:
  + // - libvpx's VP9 single thread decoder is more fuzzer friendly. It detects
  + // errors earlier than the multi-threads version.
  + // - Make peak CPU usage under control (not depending on input)
  + cfg.threads = 1;
  + (void)kMaxNumTiles4kVideo; // unused
  +#else
  // We want to use multithreading when decoding high resolution videos. But,
  // since we don't know resolution of input stream at this stage, we always
  // enable it.
  cfg.threads = std::min(number_of_cores, kMaxNumTiles4kVideo);
  +#endif
  vpx_codec_flags_t flags = 0;
  if (vpx_codec_dec_init(decoder_, vpx_codec_vp9_dx(), &cfg, flags)) {
  