Introduce (Heap)Vector trait covering zero'ed memory for unused slots.

Source/core/layout/HitTestResult.h

 /*
  * Copyright (C) 2006 Apple Computer, Inc.
  * Copyright (C) 2012 Nokia Corporation and/or its subsidiary(-ies)
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
@@ skipping 155 matching lines @@
         // determine where inside the layoutObject we hit on subsequent operations.
     RefPtrWillBeMember<Element> m_innerURLElement;
     RefPtrWillBeMember<Scrollbar> m_scrollbar;
     bool m_isOverWidget; // Returns true if we are over a widget (and not in the border/padding area of a LayoutPart for example).
 
     mutable OwnPtrWillBeMember<NodeSet> m_listBasedTestResult;
 };
 
 } // namespace blink
 
-#if ENABLE(OILPAN)
-// TODO(sof): the trait override/specialization is needed by HitTestCache's
-// HeapVector<> to handle unused slots. It is not correct for HitTestResult
-// in the wider sense of what canInitializeWithMemset provides, so fix this
-// by introducing a trait that encompasses "unused slot" handling and use it
-// here instead.
-//
-// Until that time, make this trait specialization conditional on OILPAN to
-// limit exposure.
-WTF_ALLOW_INIT_WITH_MEM_FUNCTIONS(blink::HitTestResult);
-#endif
+WTF_ALLOW_CLEAR_UNUSED_SLOTS_WITH_MEM_FUNCTIONS(blink::HitTestResult);
 
 #endif // HitTestResult_h