Add MediaGpuChannelManager::LookupChannel().

media/mojo/clients/mojo_video_decoder.h

Index: media/mojo/clients/mojo_video_decoder.h
diff --git a/media/mojo/clients/mojo_video_decoder.h b/media/mojo/clients/mojo_video_decoder.h
index 78d36c324d81a619718d5ee3f7f9ddff83be968b..bb13271206bedef4e8e009d72848cdd327a994eb 100644
--- a/media/mojo/clients/mojo_video_decoder.h
+++ b/media/mojo/clients/mojo_video_decoder.h
@@ -61,13 +61,15 @@ class MojoVideoDecoder final : public VideoDecoder,
   // Cleans up callbacks and blocks future calls.
   void Stop();
 
+  // Task runner that the decoder runs on (media thread).
   scoped_refptr<base::SingleThreadTaskRunner> task_runner_;
-  GpuVideoAcceleratorFactories* gpu_factories_;
 
   // Used to pass the remote decoder from the constructor (on the main thread)
   // to Initialize() (on the media thread).
   mojom::VideoDecoderPtrInfo remote_decoder_info_;
 
+  GpuVideoAcceleratorFactories* gpu_factories_ = nullptr;
+
   InitCB init_cb_;
   OutputCB output_cb_;
   uint64_t decode_counter_ = 0;