Flesh out the Painting API a bit.

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/Canvas.h"
+#include "sky/engine/wtf/Vector.h"
 
 namespace blink {
 class Element;
 
 class PaintingContext : public Canvas {
     DEFINE_WRAPPERTYPEINFO();
 public:
     ~PaintingContext() override;
     static PassRefPtr<PaintingContext> create(PassRefPtr<Element> element, const FloatSize& size);
 
     void commit();
 
 private:
     PaintingContext(PassRefPtr<Element> element, const FloatSize& size);
 
     RefPtr<Element> m_element;
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_PAINTING_PAINTINGCONTEXT_H_