Allow CanvasSkia to bind to an existing SkCanvas.

ui/gfx/canvas.cc

 // Copyright (c) 2010 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 "ui/gfx/canvas.h"
 
 namespace gfx {
 
 CanvasSkia* Canvas::AsCanvasSkia() {
   return NULL;
 }
 
 const CanvasSkia* Canvas::AsCanvasSkia() const {
   return NULL;
 }
 
+SkCanvas* Canvas::GetSkCanvas() {
+  return NULL;
+}
+
+const SkCanvas* Canvas::GetSkCanvas() const {
+  return NULL;
+}
+
 }  // namespace gfx;