ash: Cleanup class/struct forward declarations

ash/first_run/first_run_helper_impl.h

 // Copyright 2013 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 ASH_FIRST_RUN_FIRST_RUN_HELPER_IMPL_H_
 #define ASH_FIRST_RUN_FIRST_RUN_HELPER_IMPL_H_
 
 #include "ash/first_run/desktop_cleaner.h"
 #include "ash/first_run/first_run_helper.h"
 #include "ash/wm/overlay_event_filter.h"
 #include "base/compiler_specific.h"
 #include "base/macros.h"
 
 namespace ash {
 
-class Shell;
-
 class FirstRunHelperImpl : public FirstRunHelper,
                            public OverlayEventFilter::Delegate {
  public:
   FirstRunHelperImpl();
   ~FirstRunHelperImpl() override;
 
   // Overriden from FirstRunHelper.
   views::Widget* GetOverlayWidget() override;
   gfx::Rect GetAppListButtonBounds() override;
   void OpenTrayBubble() override;
@@ skipping 10 matching lines @@
  private:
   views::Widget* widget_;
   DesktopCleaner cleaner_;
 
   DISALLOW_COPY_AND_ASSIGN(FirstRunHelperImpl);
 };
 
 }  // namespace ash
 
 #endif  // ASH_FIRST_RUN_FIRST_RUN_HELPER_IMPL_H_