media: Cleanup class/struct forward declarations

media/gpu/v4l2_slice_video_decode_accelerator.h

 // Copyright 2015 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_V4L2_SLICE_VIDEO_DECODE_ACCELERATOR_H_
 #define MEDIA_GPU_V4L2_SLICE_VIDEO_DECODE_ACCELERATOR_H_
 
 #include <linux/videodev2.h>
 #include <stddef.h>
 #include <stdint.h>
@@ skipping 453 matching lines @@
   GetGLContextCallback get_gl_context_cb_;
   // Callback to set the correct gl context.
   MakeGLContextCurrentCallback make_context_current_cb_;
 
   // The WeakPtrFactory for |weak_this_|.
   base::WeakPtrFactory<V4L2SliceVideoDecodeAccelerator> weak_this_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(V4L2SliceVideoDecodeAccelerator);
 };
 
-class V4L2H264Picture;
-class V4L2VP8Picture;
-class V4L2VP9Picture;
-
 }  // namespace media
 
 #endif  // MEDIA_GPU_V4L2_SLICE_VIDEO_DECODE_ACCELERATOR_H_