Make HTMLCollection / NodeList backward traversal consistent with forward one

Source/core/html/HTMLCollection.h

 /*
  * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2011, 2012 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
  * version 2 of the License, or (at your option) any later version.
@@ skipping 40 matching lines @@
     bool namedPropertyQuery(const AtomicString&, ExceptionState&);
     void namedPropertyEnumerator(Vector<String>& names, ExceptionState&);
 
     // Non-DOM API
     void namedItems(const AtomicString& name, Vector<RefPtr<Element> >&) const;
     bool isEmpty() const { return m_collectionIndexCache.isEmpty(*this); }
     bool hasExactlyOneItem() const { return m_collectionIndexCache.hasExactlyOneNode(*this); }
 
     // CollectionIndexCache API.
     bool canTraverseBackward() const { return !overridesItemAfter(); }
-    Element* itemBefore(const Element* previousItem) const;
     Element* traverseToFirstElement() const;
+    Element* traverseToLastElement() const;
     Element* traverseForwardToOffset(unsigned offset, Element& currentElement, unsigned& currentOffset) const;
+    Element* traverseBackwardToOffset(unsigned offset, Element& currentElement, unsigned& currentOffset) const;
 
 protected:
     HTMLCollection(ContainerNode& base, CollectionType, ItemAfterOverrideType);
 
     class NamedItemCache {
     public:
         static PassOwnPtr<NamedItemCache> create()
         {
             return adoptPtr(new NamedItemCache);
         }
@@ skipping 65 matching lines @@
     const unsigned m_shouldOnlyIncludeDirectChildren : 1;
     mutable OwnPtr<NamedItemCache> m_namedItemCache;
     mutable CollectionIndexCache<HTMLCollection, Element> m_collectionIndexCache;
 
     friend class LiveNodeListBase;
 };
 
 } // namespace
 
 #endif