Make it possible to custom-paint without an Element.

sky/engine/core/painting/PaintingContext.h

 // Copyright 2015 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 SKY_ENGINE_CORE_PAINTING_PAINTINGCONTEXT_H_
 #define SKY_ENGINE_CORE_PAINTING_PAINTINGCONTEXT_H_
 
-#include "sky/engine/core/painting/Paint.h"
-#include "sky/engine/platform/graphics/DisplayList.h"
-#include "sky/engine/tonic/dart_wrappable.h"
-#include "sky/engine/wtf/PassRefPtr.h"
-#include "sky/engine/wtf/RefCounted.h"
+#include "sky/engine/core/painting/Canvas.h"
 
 namespace blink {
 class Element;
 
-class PaintingContext : public RefCounted<PaintingContext>, public DartWrappable {
+class PaintingContext : public Canvas {
     DEFINE_WRAPPERTYPEINFO();
 public:
     ~PaintingContext() override;
     static PassRefPtr<PaintingContext> create(PassRefPtr<Element> element, const FloatSize& size);
 
-    double height() const { return m_size.height(); }
-    double width() const { return m_size.width(); }
-
-    void drawCircle(double x, double y, double radius, Paint* paint);
     void commit();
 
 private:
     PaintingContext(PassRefPtr<Element> element, const FloatSize& size);
 
     RefPtr<Element> m_element;
-    FloatSize m_size;
-    RefPtr<DisplayList> m_displayList;
-    SkCanvas* m_canvas;
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_PAINTING_PAINTINGCONTEXT_H_