Allow CanvasSkia to bind to an existing SkCanvas.

ui/gfx/canvas_skia_paint.h

Index: ui/gfx/canvas_skia_paint.h
diff --git a/ui/gfx/canvas_skia_paint.h b/ui/gfx/canvas_skia_paint.h
index 1d67b37693f486cb0544918f7bd51d59a73487c9..d68060f82e6529bb0f20d40b2b4e5eb25946ee4d 100644
--- a/ui/gfx/canvas_skia_paint.h
+++ b/ui/gfx/canvas_skia_paint.h
@@ -12,6 +12,17 @@
 // 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;