gpu: Cleanup class/struct forward declarations

gpu/command_buffer/service/gl_context_virtual.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 GPU_COMMAND_BUFFER_SERVICE_GL_CONTEXT_VIRTUAL_H_
 #define GPU_COMMAND_BUFFER_SERVICE_GL_CONTEXT_VIRTUAL_H_
 
 #include <string>
 #include "base/callback_forward.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
 #include "gpu/gpu_export.h"
 #include "ui/gl/gl_context.h"
 
 namespace gl {
-class GPUPreference;
 class GPUTimingClient;
 class GLShareGroup;
 class GLSurface;
 }
 
 namespace gpu {
 namespace gles2 {
 class GLES2Decoder;
 }
 
@@ skipping 27 matching lines @@
 
   scoped_refptr<gl::GLContext> shared_context_;
   base::WeakPtr<gles2::GLES2Decoder> decoder_;
 
   DISALLOW_COPY_AND_ASSIGN(GLContextVirtual);
 };
 
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_SERVICE_GL_CONTEXT_VIRTUAL_H_