message: Add DCHECK in LibvpxVp8Encoder
  Ensure that frame_buffer_controller_ does not get assigned null
  by the factory.
  Cr-Commit-Position: refs/heads/master@{#27962}
diff: @@ -490,6 +490,7 @@ int LibvpxVp8Encoder::InitEncode(const VideoCodec* inst,
  Vp8TemporalLayersFactory factory;
  frame_buffer_controller_ = factory.Create(*inst);
  }
  + RTC_DCHECK(frame_buffer_controller_);
  number_of_cores_ = number_of_cores;
  timestamp_ = 0;
  