Move the Widget hierarchy to the Oilpan heap.

Source/platform/exported/WebScrollbarImpl.h

 /*
  * Copyright (C) 2012 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *     notice, this list of conditions and the following disclaimer in the
  *     documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
  * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS BE LIABLE FOR ANY
  * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
  * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
  * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON
  * ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 #ifndef WebScrollbarImpl_h
 #define WebScrollbarImpl_h
 
 #include "platform/PlatformExport.h"
+#include "platform/heap/Handle.h"
 #include "public/platform/WebScrollbar.h"
 
 namespace blink {
 
 class Scrollbar;
-class PLATFORM_EXPORT WebScrollbarImpl : public WebScrollbar {
+class PLATFORM_EXPORT WebScrollbarImpl final : public WebScrollbar {
 public:
-    explicit WebScrollbarImpl(Scrollbar*);
+    static WebScrollbarImpl* create(Scrollbar* scrollbar)
+    {
+        return new WebScrollbarImpl(scrollbar);
+    }
 
     // Implement WebScrollbar methods
     virtual bool isOverlay() const override;
     virtual int value() const override;
     virtual WebPoint location() const override;
     virtual WebSize size() const override;
     virtual bool enabled() const override;
     virtual int maximum() const override;
     virtual int totalSize() const override;
     virtual bool isScrollableAreaActive() const override;
     virtual void getTickmarks(WebVector<WebRect>& tickmarks) const override;
     virtual ScrollbarControlSize controlSize() const override;
     virtual ScrollbarPart pressedPart() const override;
     virtual ScrollbarPart hoveredPart() const override;
     virtual ScrollbarOverlayStyle scrollbarOverlayStyle() const override;
     virtual bool isCustomScrollbar() const override;
     virtual Orientation orientation() const override;
     virtual bool isLeftSideVerticalScrollbar() const override;
     virtual bool isAlphaLocked() const override;
     virtual void setIsAlphaLocked(bool) override;
 
 private:
-    RefPtr<Scrollbar> m_scrollbar;
+    explicit WebScrollbarImpl(Scrollbar*);
+
+    RefPtrWillBePersistent<Scrollbar> m_scrollbar;
 };
 
 } // namespace blink
 
 #endif