Move the Widget hierarchy to the Oilpan heap.

Source/core/rendering/RenderScrollbar.h

 /*
  * Copyright (C) 2008, 2009 Apple 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. ``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
  * 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 RenderScrollbar_h
 #define RenderScrollbar_h
 
 #include "core/rendering/style/RenderStyleConstants.h"
+#include "platform/heap/Handle.h"
 #include "platform/scroll/Scrollbar.h"
 #include "wtf/HashMap.h"
 
 namespace blink {
 
 class LocalFrame;
 class Node;
 class RenderBox;
 class RenderScrollbarPart;
 class RenderStyle;
 
 class RenderScrollbar final : public Scrollbar {
-protected:
-    RenderScrollbar(ScrollableArea*, ScrollbarOrientation, Node*, LocalFrame*);
-
+#if ENABLE(OILPAN)
+    USING_PRE_FINALIZER(RenderScrollbar, destroyParts);
+#endif
 public:
-    friend class Scrollbar;
-    static PassRefPtr<Scrollbar> createCustomScrollbar(ScrollableArea*, ScrollbarOrientation, Node*, LocalFrame* owningFrame = 0);
+    static PassRefPtrWillBeRawPtr<Scrollbar> createCustomScrollbar(ScrollableArea*, ScrollbarOrientation, Node*, LocalFrame* owningFrame = 0);
     virtual ~RenderScrollbar();
 
     RenderBox* owningRenderer() const;
 
     IntRect buttonRect(ScrollbarPart);
     IntRect trackRect(int startLength, int endLength);
     IntRect trackPieceRectWithMargins(ScrollbarPart, const IntRect&);
 
     int minimumThumbLength();
 
     virtual bool isOverlayScrollbar() const override { return false; }
 
     RenderScrollbarPart* getPart(ScrollbarPart partType) { return m_parts.get(partType); }
 
+    virtual void trace(Visitor*) override;
+
+protected:
+    RenderScrollbar(ScrollableArea*, ScrollbarOrientation, Node*, LocalFrame*);
+
 private:
+    friend class Scrollbar;
+
     virtual void setParent(Widget*) override;
     virtual void setEnabled(bool) override;
 
     virtual void setHoveredPart(ScrollbarPart) override;
     virtual void setPressedPart(ScrollbarPart) override;
 
     virtual void styleChanged() override;
 
     virtual bool isCustomScrollbar() const override { return true; }
 
     void updateScrollbarParts(bool destroy = false);
 
     PassRefPtr<RenderStyle> getScrollbarPseudoStyle(ScrollbarPart, PseudoId);
     void updateScrollbarPart(ScrollbarPart, bool destroy = false);
 
+    void destroyParts();
+
     // This Scrollbar(Widget) may outlive the DOM which created it (during tear down),
     // so we keep a reference to the Node which caused this custom scrollbar creation.
     // This will not create a reference cycle as the Widget tree is owned by our containing
     // FrameView which this Node pointer can in no way keep alive. See webkit bug 80610.
-    RefPtrWillBePersistent<Node> m_owner;
+    RefPtrWillBeMember<Node> m_owner;
 
-    LocalFrame* m_owningFrame;
-    WillBePersistentHeapHashMap<unsigned, RawPtrWillBeMember<RenderScrollbarPart> > m_parts;
+    RawPtrWillBeMember<LocalFrame> m_owningFrame;
+
+    WillBeHeapHashMap<unsigned, RawPtrWillBeMember<RenderScrollbarPart> > m_parts;
+
+#if ENABLE(OILPAN)
+    // Flag set during finalization to turn off updating of the above heap hash map.
+    bool m_updatePartsMap;
+#endif
 };
 
 DEFINE_TYPE_CASTS(RenderScrollbar, ScrollbarThemeClient, scrollbar, scrollbar->isCustomScrollbar(), scrollbar.isCustomScrollbar());
 
 } // namespace blink
 
 #endif // RenderScrollbar_h