Make HTMLCollection / NodeList backward traversal consistent with forward one

Source/core/dom/ChildNodeList.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, 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 30 matching lines @@
     virtual ~ChildNodeList();
 
     // DOM API.
     virtual unsigned length() const OVERRIDE { return m_collectionIndexCache.nodeCount(*this); }
     virtual Node* item(unsigned index) const OVERRIDE { return m_collectionIndexCache.nodeAt(*this, index); }
 
     // Non-DOM API.
     void invalidateCache() { m_collectionIndexCache.invalidate(); }
     ContainerNode& ownerNode() const { return *m_parent; }
 
+    ContainerNode& rootNode() const { return ownerNode(); }
+
     // CollectionIndexCache API.
-    ContainerNode& rootNode() const { return ownerNode(); }
     bool canTraverseBackward() const { return true; }
-    Node* itemBefore(const Node* previousItem) const;
     Node* traverseToFirstElement() const { return rootNode().firstChild(); }
+    Node* traverseToLastElement() const { return rootNode().lastChild(); }
     Node* traverseForwardToOffset(unsigned offset, Node& currentNode, unsigned& currentOffset) const;
+    Node* traverseBackwardToOffset(unsigned offset, Node& currentNode, unsigned& currentOffset) const;
 
 private:
     explicit ChildNodeList(ContainerNode& rootNode);
 
     virtual bool isChildNodeList() const OVERRIDE { return true; }
     virtual Node* virtualOwnerNode() const OVERRIDE;
 
     RefPtr<ContainerNode> m_parent;
     mutable CollectionIndexCache<ChildNodeList, Node> m_collectionIndexCache;
 };
 
 DEFINE_TYPE_CASTS(ChildNodeList, NodeList, nodeList, nodeList->isChildNodeList(), nodeList.isChildNodeList());
 
 } // namespace WebCore
 
 #endif // ChildNodeList_h