media: Cleanup class/struct forward declarations

media/gpu/ipc/service/media_gpu_channel_manager.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_GPU_IPC_SERVICE_MEDIA_GPU_CHANNEL_MANAGER_H_
 #define MEDIA_GPU_IPC_SERVICE_MEDIA_GPU_CHANNEL_MANAGER_H_
 
 #include <stdint.h>
 
 #include <memory>
 
 #include "base/containers/scoped_ptr_hash_map.h"
 #include "base/macros.h"
 #include "ipc/ipc_listener.h"
 #include "ipc/ipc_sender.h"
 #include "media/video/video_decode_accelerator.h"
 
 namespace gpu {
-class GpuChannel;
 class GpuChannelManager;
 }
 
 namespace media {
 
 class MediaGpuChannel;
 
 class MediaGpuChannelManager {
  public:
   explicit MediaGpuChannelManager(gpu::GpuChannelManager* channel_manager);
   ~MediaGpuChannelManager();
 
   void AddChannel(int32_t client_id);
   void RemoveChannel(int32_t client_id);
   void DestroyAllChannels();
 
  private:
   gpu::GpuChannelManager* const channel_manager_;
   base::ScopedPtrHashMap<int32_t, std::unique_ptr<MediaGpuChannel>>
       media_gpu_channels_;
   DISALLOW_COPY_AND_ASSIGN(MediaGpuChannelManager);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_GPU_IPC_SERVICE_MEDIA_GPU_CHANNEL_MANAGER_H_