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

ash/desktop_background/desktop_background_view.h

Index: ash/desktop_background/desktop_background_view.h
diff --git a/ash/desktop_background/desktop_background_view.h b/ash/desktop_background/desktop_background_view.h
index 6af658325a6423f13ad9e61ed3b0852a91ba986c..76efa23b4e13ce68b6800f169fe2dcc30f0d4f81 100644
--- a/ash/desktop_background/desktop_background_view.h
+++ b/ash/desktop_background/desktop_background_view.h
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium Authors. All rights reserved.
+// 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.
 
@@ -20,7 +20,7 @@ class DesktopBackgroundView : public views::WidgetDelegateView {
 
  private:
   // Overridden from views::View:
-  virtual void OnPaint(gfx::Canvas* canvas) OVERRIDE;
+  virtual void OnPaint(gfx::CanvasSkia* canvas) OVERRIDE;
   virtual bool OnMousePressed(const views::MouseEvent& event) OVERRIDE;
   virtual void OnMouseReleased(const views::MouseEvent& event) OVERRIDE;