Oilpan: Make sure that vtables for garbage collected mixin objects have

Source/core/dom/LiveNodeListBase.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  *           (C) 2001 Dirk Mueller (mueller@kde.org)
  * Copyright (C) 2004, 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2014 Samsung Electronics. All rights reserved.
  *
  * 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
@@ skipping 76 matching lines @@
     static Element* lastMatchingElement(const NodeListType&);
     template <class NodeListType>
     static Element* nextMatchingElement(const NodeListType&, Element& current);
     template <class NodeListType>
     static Element* previousMatchingElement(const NodeListType&, Element& current);
     template <class NodeListType>
     static Element* traverseMatchingElementsForwardToOffset(const NodeListType&, unsigned offset, Element& currentElement, unsigned& currentOffset);
     template <class NodeListType>
     static Element* traverseMatchingElementsBackwardToOffset(const NodeListType&, unsigned offset, Element& currentElement, unsigned& currentOffset);
 
-    void trace(Visitor* visitor) { visitor->trace(m_ownerNode); }
+    virtual void trace(Visitor* visitor) { visitor->trace(m_ownerNode); }
 
 private:
     RefPtrWillBeMember<ContainerNode> m_ownerNode; // Cannot be null.
     const unsigned m_rootType : 1;
     const unsigned m_invalidationType : 4;
     const unsigned m_collectionType : 5;
 };
 
 ALWAYS_INLINE bool LiveNodeListBase::shouldInvalidateTypeOnAttributeChange(NodeListInvalidationType type, const QualifiedName& attrName)
 {
@@ skipping 79 matching lines @@
     for (Element* previous = previousMatchingElement(nodeList, currentElement); previous; previous = previousMatchingElement(nodeList, *previous)) {
         if (--currentOffset == offset)
             return previous;
     }
     return 0;
 }
 
 } // namespace WebCore
 
 #endif // LiveNodeListBase_h