ui: Cleanup class/struct forward declarations

ui/native_theme/native_theme_base.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_NATIVE_THEME_NATIVE_THEME_BASE_H_
 #define UI_NATIVE_THEME_NATIVE_THEME_BASE_H_
 
 #include <memory>
 
 #include "base/compiler_specific.h"
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "skia/ext/platform_canvas.h"
 #include "ui/native_theme/native_theme.h"
 
 namespace gfx {
-class Canvas;
-class ImageSkia;
 class Rect;
 class Size;
 }
 
 namespace ui {
 
 // Theme support for non-Windows toolkits.
 class NATIVE_THEME_EXPORT NativeThemeBase : public NativeTheme {
  public:
   // NativeTheme implementation:
@@ skipping 162 matching lines @@
 
   // The length of the arrow buttons, 0 means no buttons are drawn.
   int scrollbar_button_length_;
 
   DISALLOW_COPY_AND_ASSIGN(NativeThemeBase);
 };
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_NATIVE_THEME_BASE_H_