Add APIs for dynamic display lists, using SkDrawable and SkPictureRecorder::endRecordingAsDrawable.

sky/engine/core/painting/Picture.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_PICTURE_H_
 #define SKY_ENGINE_CORE_PAINTING_PICTURE_H_
 
 #include "sky/engine/tonic/dart_wrappable.h"
 #include "sky/engine/wtf/PassRefPtr.h"
 #include "sky/engine/wtf/RefCounted.h"
 #include "third_party/skia/include/core/SkPicture.h"
 
 namespace blink {
 
 class Picture : public RefCounted<Picture>, public DartWrappable {
     DEFINE_WRAPPERTYPEINFO();
 public:
     ~Picture() override;
     static PassRefPtr<Picture> create(PassRefPtr<SkPicture> skPicture);
 
     SkPicture* toSkia() const { return m_picture.get(); }
 
 private:
-    Picture(PassRefPtr<SkPicture> skPicture);
+    explicit Picture(PassRefPtr<SkPicture> skPicture);
 
     RefPtr<SkPicture> m_picture;
 };
 
 } // namespace blink
 
 #endif  // SKY_ENGINE_CORE_PAINTING_PICTURE_H_