Allow CanvasSkia to bind to an existing SkCanvas.

ui/aura/demo/demo_main.cc

 // Copyright (c) 2011 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.
 
 #include "base/at_exit.h"
 #include "base/command_line.h"
 #include "base/i18n/icu_util.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop.h"
 #include "third_party/skia/include/core/SkXfermode.h"
@@ skipping 33 matching lines @@
     return HTCAPTION;
   }
   virtual bool OnMouseEvent(aura::MouseEvent* event) OVERRIDE {
     return true;
   }
   virtual bool ShouldActivate(aura::MouseEvent* event) OVERRIDE { return true; }
   virtual void OnActivated() OVERRIDE {}
   virtual void OnLostActive() OVERRIDE {}
   virtual void OnCaptureLost() OVERRIDE {}
   virtual void OnPaint(gfx::Canvas* canvas) OVERRIDE {
-    canvas->AsCanvasSkia()->drawColor(color_, SkXfermode::kSrc_Mode);
+    canvas->GetSkCanvas()->drawColor(color_, SkXfermode::kSrc_Mode);
   }
   virtual void OnWindowDestroying() OVERRIDE {}
   virtual void OnWindowDestroyed() OVERRIDE {}
 
  private:
   SkColor color_;
 
   DISALLOW_COPY_AND_ASSIGN(DemoWindowDelegate);
 };
 
@@ skipping 41 matching lines @@
   window3.set_id(3);
   window3.Init();
   window3.SetBounds(gfx::Rect(10, 10, 50, 50));
   window3.Show();
   window3.SetParent(&window2);
 
   aura::Desktop::GetInstance()->Run();
   return 0;
 }