Move the Widget hierarchy to the Oilpan heap.

Source/core/accessibility/AXScrollbar.h

Index: Source/core/accessibility/AXScrollbar.h
diff --git a/Source/core/accessibility/AXScrollbar.h b/Source/core/accessibility/AXScrollbar.h
index 1c65101547ca13083b88cd92cdded647d1698897..df1448ca66f5f968572e263678ac1b01cc69a7fa 100644
--- a/Source/core/accessibility/AXScrollbar.h
+++ b/Source/core/accessibility/AXScrollbar.h
@@ -61,7 +61,7 @@ private:
     virtual void setValue(float) OVERRIDE;
     virtual float valueForRange() const OVERRIDE;
 
-    RefPtr<Scrollbar> m_scrollbar;
+    RefPtrWillBePersistent<Scrollbar> m_scrollbar;

[haraken, 2014/09/29 14:16:35]

Hmm, it seems that the fact that AXScrollbar can hold a RefPtrWillBePersistent
to the corresponding Scrollbar implies that AXScrollView can hold a
RefPtrWillBePersistent to the corresponding ScrollView. I wonder why
AXScrollbar's RefPtrWillBePersistent is OK but AXScrollView's
RefPtrWillBePersistent is not OK.

[sof, 2014/10/08 11:52:25]

My reasoning was simply that Scrollbar is a leaf object, and FrameView isn't.
The risk of leak creation mirrors that, to some degree.

I suggest we keep it as a persistent while we wait for the AXObject hierarchy to
be moved to the heap.

 };
 
 DEFINE_AX_OBJECT_TYPE_CASTS(AXScrollbar, isAXScrollbar());