Move the Widget hierarchy to the Oilpan heap.

Source/platform/scroll/ScrollView.h

Index: Source/platform/scroll/ScrollView.h
diff --git a/Source/platform/scroll/ScrollView.h b/Source/platform/scroll/ScrollView.h
index 1a63213971e32ccc22afab426aa87c5d78822d9b..62f9a28613e90c8747a2e4fe39ef01cf89028b77 100644
--- a/Source/platform/scroll/ScrollView.h
+++ b/Source/platform/scroll/ScrollView.h
@@ -30,6 +30,7 @@
 #include "platform/PlatformExport.h"
 #include "platform/Widget.h"
 #include "platform/geometry/IntRect.h"
+#include "platform/heap/Handle.h"
 #include "platform/scroll/ScrollTypes.h"
 #include "platform/scroll/ScrollableArea.h"
 #include "platform/scroll/Scrollbar.h"
@@ -45,6 +46,8 @@ class PLATFORM_EXPORT ScrollView : public Widget, public ScrollableArea {
 public:
     virtual ~ScrollView();
 
+    virtual void trace(Visitor*) OVERRIDE;
+
     // ScrollableArea functions.
     virtual int scrollSize(ScrollbarOrientation) const override;
     virtual void setScrollOffset(const IntPoint&) override;
@@ -63,9 +66,11 @@ public:
     // Returns a clip rect in host window coordinates. Used to clip the blit on a scroll.
     virtual IntRect windowClipRect(IncludeScrollbarsInRect = ExcludeScrollbars) const = 0;
 
+    typedef WillBeHeapHashSet<RefPtrWillBeMember<Widget> > ChildrenWidgetSet;
+
     // Functions for child manipulation and inspection.
-    const HashSet<RefPtr<Widget> >* children() const { return &m_children; }
-    virtual void addChild(PassRefPtr<Widget>);
+    const ChildrenWidgetSet* children() const { return &m_children; }
+    virtual void addChild(PassRefPtrWillBeRawPtr<Widget>);
     virtual void removeChild(Widget*);
 
     // If the scroll view does not use a native widget, then it will have cross-platform Scrollbars. These functions
@@ -104,7 +109,7 @@ public:
     void setClipsRepaints(bool);
 
     // Overridden by FrameView to create custom CSS scrollbars if applicable.
-    virtual PassRefPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
+    virtual PassRefPtrWillBeRawPtr<Scrollbar> createScrollbar(ScrollbarOrientation);
 
     // The visible content rect has a location that is the scrolled offset of the document. The width and height are the viewport width
     // and height. By default the scrollbars themselves are excluded from this rectangle, but an optional boolean argument allows them to be
@@ -313,15 +318,15 @@ private:
     // some cleanup.
     void setScrollOffsetFromUpdateScrollbars(const IntSize&);
 
-    RefPtr<Scrollbar> m_horizontalScrollbar;
-    RefPtr<Scrollbar> m_verticalScrollbar;
+    RefPtrWillBeMember<Scrollbar> m_horizontalScrollbar;
+    RefPtrWillBeMember<Scrollbar> m_verticalScrollbar;
     ScrollbarMode m_horizontalScrollbarMode;
     ScrollbarMode m_verticalScrollbarMode;
 
     bool m_horizontalScrollbarLock;
     bool m_verticalScrollbarLock;
 
-    HashSet<RefPtr<Widget> > m_children;
+    ChildrenWidgetSet m_children;
 
     DoubleSize m_pendingScrollDelta;
     DoublePoint m_scrollPosition;