Oilpan: fix build after r197063, move HitTestCache to the heap.

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 16 matching lines @@
 #include "core/layout/HitTestRequest.h"
 #include "platform/geometry/FloatQuad.h"
 #include "platform/geometry/FloatRect.h"
 #include "platform/geometry/LayoutRect.h"
 #include "platform/heap/Handle.h"
 #include "platform/text/TextDirection.h"
 #include "wtf/Forward.h"
 #include "wtf/ListHashSet.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/RefPtr.h"
+#include "wtf/VectorTraits.h"
 
 namespace blink {
 
 class Element;
 class LocalFrame;
 class HTMLAreaElement;
 class HTMLMediaElement;
 class Image;
 class KURL;
 class Node;
@@ skipping 118 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
+
 #endif // HitTestResult_h