Move the Widget hierarchy to the Oilpan heap.

Source/core/accessibility/AXScrollbar.h

 /*
  * Copyright (C) 2010 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
@@ skipping 43 matching lines @@
     virtual AccessibilityRole roleValue() const OVERRIDE { return ScrollBarRole; }
     virtual AccessibilityOrientation orientation() const OVERRIDE;
     virtual Document* document() const OVERRIDE;
 
     virtual bool isEnabled() const OVERRIDE;
 
     // Assumes float [0..1]
     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());
 
 } // namespace blink
 
 #endif // AXScrollbar_h