Make HTMLCollection / NodeList backward traversal consistent with forward one

Source/core/dom/LiveNodeList.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.
  *
  * 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.
@@ skipping 32 matching lines @@
 
     virtual unsigned length() const OVERRIDE FINAL { return m_collectionIndexCache.nodeCount(*this); }
     virtual Node* item(unsigned offset) const OVERRIDE FINAL { return m_collectionIndexCache.nodeAt(*this, offset); }
     virtual bool elementMatches(const Element&) const = 0;
 
     virtual void invalidateCache(Document* oldDocument) const OVERRIDE FINAL;
     bool shouldOnlyIncludeDirectChildren() const { return false; }
 
     // Collection IndexCache API.
     bool canTraverseBackward() const { return true; }
-    Element* itemBefore(const Element* previousItem) const;
     Element* traverseToFirstElement() const;
+    Element* traverseToLastElement() const;
     Element* traverseForwardToOffset(unsigned offset, Element& currentNode, unsigned& currentOffset) const;
+    Element* traverseBackwardToOffset(unsigned offset, Element& currentNode, unsigned& currentOffset) const;
 
 private:
     virtual Node* virtualOwnerNode() const OVERRIDE FINAL;
 
     mutable CollectionIndexCache<LiveNodeList, Element> m_collectionIndexCache;
 };
 
 } // namespace WebCore
 
 #endif // LiveNodeList_h