Make HTMLCollection / NodeList backward traversal consistent with forward one

Source/core/dom/ChildNodeList.cpp

 /*
  * 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, 2008 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 26 matching lines @@
 Node* ChildNodeList::virtualOwnerNode() const
 {
     return &ownerNode();
 }
 
 ChildNodeList::~ChildNodeList()
 {
     m_parent->nodeLists()->removeChildNodeList(this);
 }
 
-Node* ChildNodeList::itemBefore(const Node* previous) const
-{
-    return LIKELY(!!previous) ? previous->previousSibling() : rootNode().lastChild();
-}
-
 Node* ChildNodeList::traverseForwardToOffset(unsigned offset, Node& currentNode, unsigned& currentOffset) const
 {
     ASSERT(currentOffset < offset);
     Node* next = &currentNode;
     while ((next = next->nextSibling())) {
         if (++currentOffset == offset)
             return next;
     }
     return 0;
 }
 
+Node* ChildNodeList::traverseBackwardToOffset(unsigned offset, Node& currentNode, unsigned& currentOffset) const
+{
+    ASSERT(currentOffset > offset);
+    Node* previous = &currentNode;
+    while ((previous = previous->previousSibling())) {
+        if (--currentOffset == offset)
+            return previous;
+    }
+    return 0;
+}
+
 } // namespace WebCore