extensions: Cleanup class/struct fwd declarations

extensions/renderer/guest_view/extensions_guest_view_container.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 EXTENSIONS_RENDERER_GUEST_VIEW_EXTENSIONS_GUEST_VIEW_CONTAINER_H_
 #define EXTENSIONS_RENDERER_GUEST_VIEW_EXTENSIONS_GUEST_VIEW_CONTAINER_H_
 
 #include <queue>
 
 #include "base/macros.h"
 #include "components/guest_view/renderer/guest_view_container.h"
 #include "v8/include/v8.h"
 
-namespace gfx {
-class Size;
-}
-
 namespace extensions {
 
 class ExtensionsGuestViewContainer : public guest_view::GuestViewContainer {
  public:
   explicit ExtensionsGuestViewContainer(content::RenderFrame* render_frame);
 
  protected:
   ~ExtensionsGuestViewContainer() override;
 
  private:
   // GuestViewContainer implementation.
   void OnDestroy(bool embedder_frame_destroyed) override;
 
   DISALLOW_COPY_AND_ASSIGN(ExtensionsGuestViewContainer);
 };
 
 }  // namespace extensions
 
 #endif  // EXTENSIONS_RENDERER_GUEST_VIEW_EXTENSIONS_GUEST_VIEW_CONTAINER_H_