media: Cleanup class/struct forward declarations

media/filters/ffmpeg_video_decoder.h

 // Copyright (c) 2012 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_FILTERS_FFMPEG_VIDEO_DECODER_H_
 #define MEDIA_FILTERS_FFMPEG_VIDEO_DECODER_H_
 
 #include <list>
 #include <memory>
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/threading/thread_checker.h"
 #include "media/base/video_decoder.h"
 #include "media/base/video_decoder_config.h"
 #include "media/base/video_frame_pool.h"
 #include "media/ffmpeg/ffmpeg_deleters.h"
 
 struct AVCodecContext;
 struct AVFrame;
 
-namespace base {
-class SingleThreadTaskRunner;
-}
-
 namespace media {
 
 class DecoderBuffer;
 
 class MEDIA_EXPORT FFmpegVideoDecoder : public VideoDecoder {
  public:
   static bool IsCodecSupported(VideoCodec codec);
 
   FFmpegVideoDecoder();
   ~FFmpegVideoDecoder() override;
@@ skipping 55 matching lines @@
   VideoFramePool frame_pool_;
 
   bool decode_nalus_;
 
   DISALLOW_COPY_AND_ASSIGN(FFmpegVideoDecoder);
 };
 
 }  // namespace media
 
 #endif  // MEDIA_FILTERS_FFMPEG_VIDEO_DECODER_H_