gpu: Cleanup class/struct forward declarations

gpu/command_buffer/service/mailbox_manager_impl.h

 // Copyright 2014 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_MAILBOX_MANAGER_IMPL_H_
 #define GPU_COMMAND_BUFFER_SERVICE_MAILBOX_MANAGER_IMPL_H_
 
 #include <map>
 #include <utility>
 
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "gpu/command_buffer/common/constants.h"
 #include "gpu/command_buffer/common/mailbox.h"
 #include "gpu/command_buffer/service/mailbox_manager.h"
 #include "gpu/gpu_export.h"
 
 namespace gpu {
 namespace gles2 {
 
 class TextureBase;
-class TextureManager;
 
 // Manages resources scoped beyond the context or context group level.
 class GPU_EXPORT MailboxManagerImpl : public MailboxManager {
  public:
   MailboxManagerImpl();
 
   // MailboxManager implementation:
   TextureBase* ConsumeTexture(const Mailbox& mailbox) override;
   void ProduceTexture(const Mailbox& mailbox, TextureBase* texture) override;
   bool UsesSync() override;
@@ skipping 21 matching lines @@
   TextureToMailboxMap textures_to_mailboxes_;
 
   DISALLOW_COPY_AND_ASSIGN(MailboxManagerImpl);
 };
 
 }  // namespage gles2
 }  // namespace gpu
 
 #endif  // GPU_COMMAND_BUFFER_SERVICE_MAILBOX_MANAGER_IMPL_H_