ui: Cleanup class/struct forward declarations

ui/views/widget/widget.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_WIDGET_WIDGET_H_
 #define UI_VIEWS_WIDGET_WIDGET_H_
 
 #include <map>
 #include <memory>
 #include <set>
@@ skipping 25 matching lines @@
 #if defined(CreateWindow)
 #undef CreateWindow
 #endif
 #endif
 
 namespace base {
 class TimeDelta;
 }
 
 namespace gfx {
-class Canvas;
 class Point;
 class Rect;
 }
 
 namespace ui {
 class Accelerator;
 class Compositor;
 class DefaultThemeProvider;
 class InputMethod;
 class Layer;
@@ skipping 915 matching lines @@
   bool movement_disabled_;
 
   ScopedObserver<ui::NativeTheme, ui::NativeThemeObserver> observer_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(Widget);
 };
 
 }  // namespace views
 
 #endif  // UI_VIEWS_WIDGET_WIDGET_H_