Move the Widget hierarchy to the Oilpan heap.

Source/core/rendering/RenderScrollbar.cpp

 /*
  * 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
@@ skipping 18 matching lines @@
 #include "core/css/PseudoStyleRequest.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
 #include "core/rendering/RenderPart.h"
 #include "core/rendering/RenderScrollbarPart.h"
 #include "core/rendering/RenderScrollbarTheme.h"
 #include "platform/graphics/GraphicsContext.h"
 
 namespace blink {
 
-PassRefPtr<Scrollbar> RenderScrollbar::createCustomScrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, Node* ownerNode, LocalFrame* owningFrame)
+PassRefPtrWillBeRawPtr<Scrollbar> RenderScrollbar::createCustomScrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, Node* ownerNode, LocalFrame* owningFrame)
 {
-    return adoptRef(new RenderScrollbar(scrollableArea, orientation, ownerNode, owningFrame));
+    return adoptRefWillBeNoop(new RenderScrollbar(scrollableArea, orientation, ownerNode, owningFrame));
 }
 
 RenderScrollbar::RenderScrollbar(ScrollableArea* scrollableArea, ScrollbarOrientation orientation, Node* ownerNode, LocalFrame* owningFrame)
     : Scrollbar(scrollableArea, orientation, RegularScrollbar, RenderScrollbarTheme::renderScrollbarTheme())
     , m_owner(ownerNode)
     , m_owningFrame(owningFrame)
 {
     ASSERT(ownerNode || owningFrame);
 
     // FIXME: We need to do this because RenderScrollbar::styleChanged is called as soon as the scrollbar is created.
@@ skipping 15 matching lines @@
 }
 
 RenderScrollbar::~RenderScrollbar()
 {
     if (!m_parts.isEmpty()) {
         // When a scrollbar is detached from its parent (causing all parts removal) and
         // ready to be destroyed, its destruction can be delayed because of RefPtr
         // maintained in other classes such as EventHandler (m_lastScrollbarUnderMouse).
         // Meanwhile, we can have a call to updateScrollbarPart which recreates the
         // scrollbar part. So, we need to destroy these parts since we don't want them
         // to call on a destroyed scrollbar. See webkit bug 68009.

[haraken, 2014/09/29 14:16:36]

The culprit of the complexity is this bug. How hard do you think to fix the bug?

[sof, 2014/10/02 14:03:54]

Someone with rendering knowledge is better qualified to answer that. Much better
:)

+        //
+        // Oilpan: see comment next to m_parts declaration why this is
+        // safe to do and required.
         updateScrollbarParts(true);
     }
 }
 
+void RenderScrollbar::trace(Visitor* visitor)
+{
+#if ENABLE(OILPAN)
+    visitor->trace(m_owner);
+    visitor->trace(m_owningFrame);
+#endif
+    Scrollbar::trace(visitor);
+}
+
 RenderBox* RenderScrollbar::owningRenderer() const
 {
     if (m_owningFrame) {
         RenderBox* currentRenderer = m_owningFrame->ownerRenderer();
         return currentRenderer;
     }
     return m_owner && m_owner->renderer() ? m_owner->renderer()->enclosingBox() : 0;
 }
 
 void RenderScrollbar::setParent(Widget* parent)
@@ skipping 250 matching lines @@
 int RenderScrollbar::minimumThumbLength()
 {
     RenderScrollbarPart* partRenderer = m_parts.get(ThumbPart);
     if (!partRenderer)
         return 0;
     partRenderer->layout();
     return orientation() == HorizontalScrollbar ? partRenderer->width() : partRenderer->height();
 }
 
 }