Allow CanvasSkia to bind to an existing SkCanvas.

ui/gfx/canvas_skia_paint.h

 // Copyright (c) 2009 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_SKIA_PAINT_H_
 #define UI_GFX_CANVAS_SKIA_PAINT_H_
 #pragma once
 
 #include "skia/ext/canvas_paint.h"
 #include "ui/gfx/canvas_skia.h"
 
 // Define a gfx::CanvasSkiaPaint type that wraps our gfx::Canvas like the
 // skia::PlatformCanvasPaint wraps PlatformCanvas.
 
+namespace skia {
+
+template<> inline
+PlatformCanvas* GetPlatformCanvas(skia::CanvasPaintT<gfx::CanvasSkia>* canvas) {
+  PlatformCanvas* platform_canvas = canvas->platform_canvas();
+  DCHECK(platform_canvas);
+  return platform_canvas;
+}
+
+}  // namespace skia
+
 namespace gfx {
 
 typedef skia::CanvasPaintT<CanvasSkia> CanvasSkiaPaint;
 
 }  // namespace gfx
 
 #endif  // UI_GFX_CANVAS_SKIA_PAINT_H_