ui/gfx: Make gfx::Canvas inherit from gfx::CanvasSkia.

ui/gfx/canvas_paint.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_GFX_CANVAS_PAINT_H_
 #define UI_GFX_CANVAS_PAINT_H_
 #pragma once
 
 #include "ui/base/ui_export.h"
 #include "ui/gfx/native_widget_types.h"
 
 namespace gfx {
 
-class Canvas;
+class CanvasSkia;
 class Rect;
 
 class CanvasPaint {
  public:
   // Creates a canvas that paints to |view| when it is destroyed. The canvas is
   // sized to the client area of |view|.
   UI_EXPORT static CanvasPaint* CreateCanvasPaint(gfx::NativeView view);
 
   virtual ~CanvasPaint() {}
 
   // Returns true if the canvas has an invalid rect that needs to be repainted.
   virtual bool IsValid() const = 0;
 
   // Returns the rectangle that is invalid.
   virtual gfx::Rect GetInvalidRect() const = 0;
 
   // Returns the underlying Canvas.
-  virtual Canvas* AsCanvas() = 0;
+  virtual CanvasSkia* AsCanvas() = 0;
 };
 
 }  // namespace gfx
 
 #endif  // UI_GFX_CANVAS_PAINT_H_