ui: Cleanup class/struct forward declarations

ui/views/controls/slider.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 UI_VIEWS_CONTROLS_SLIDER_H_
 #define UI_VIEWS_CONTROLS_SLIDER_H_
 
 #include "base/macros.h"
 #include "ui/gfx/animation/animation_delegate.h"
 #include "ui/views/view.h"
 #include "ui/views/views_export.h"
 
 typedef unsigned int SkColor;
 
 namespace gfx {
-class ImageSkia;
 class SlideAnimation;
 }
 
 namespace views {
 
 namespace test {
 class SliderTestApi;
 }
 
 class Slider;
@@ skipping 118 matching lines @@
   // Relative position of the mouse cursor (or the touch point) on the slider's
   // button.
   int initial_button_offset_;
 
   DISALLOW_COPY_AND_ASSIGN(Slider);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_CONTROLS_SLIDER_H_