ui: Cleanup class/struct forward declarations

ui/native_theme/common_theme.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_COMMON_THEME_H_
 #define UI_NATIVE_THEME_COMMON_THEME_H_
 
 #include <memory>
 
 #include "ui/native_theme/native_theme.h"
 
 class SkCanvas;
 
-namespace gfx {
-class Canvas;
-}
-
 namespace ui {
 
 // Drawing code that is common for all platforms.
 
 // Returns the color to use on Aura for |color_id|.  For a few colors that are
 // theme-specific, |base_theme| must be non-null; consult the code to see which
 // color IDs fall into this category.
 SkColor NATIVE_THEME_EXPORT GetAuraColor(NativeTheme::ColorId color_id,
                                          const NativeTheme* base_theme);
 
 void NATIVE_THEME_EXPORT CommonThemePaintMenuItemBackground(
     const NativeTheme* theme,
     SkCanvas* canvas,
     NativeTheme::State state,
     const gfx::Rect& rect,
     const NativeTheme::MenuItemExtraParams& menu_item);
 
 }  // namespace ui
 
 #endif  // UI_NATIVE_THEME_COMMON_THEME_H_