Port SiblingTraversalStrategies to ElementTraversal API

Source/core/dom/ElementTraversal.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, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2009 Torch Mobile Inc. All rights reserved. (http://www.torchmobile.com/)
  *
  * 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 38 matching lines @@
 Element* nextSkippingChildren(const ContainerNode&, const Node* stayWithin);
 
 // Pre-order traversal including the pseudo-elements.
 Element* previousIncludingPseudo(const Node&, const Node* stayWithin = 0);
 Element* nextIncludingPseudo(const Node&, const Node* stayWithin = 0);
 Element* nextIncludingPseudoSkippingChildren(const Node&, const Node* stayWithin = 0);
 
 // Utility function to traverse only the element and pseudo-element siblings of a node.
 Element* pseudoAwarePreviousSibling(const Node&);
 
-// Next sibling.
+// Previous / Next sibling.
+Element* previousSibling(const Node&);
 Element* nextSibling(const Node&);
 
 template <class NodeType>
 inline Element* firstElementWithinTemplate(NodeType& current)
 {
     // Except for the root containers, only elements can have element children.
     Node* node = current.firstChild();
     while (node && !node->isElementNode())
         node = node->nextSibling();
     return toElement(node);
@@ skipping 70 matching lines @@
 }
 
 inline Element* pseudoAwarePreviousSibling(const Node& current)
 {
     Node* node = current.pseudoAwarePreviousSibling();
     while (node && !node->isElementNode())
         node = node->pseudoAwarePreviousSibling();
     return toElement(node);
 }
 
+inline Element* previousSibling(const Node& current)
+{
+    Node* node = current.previousSibling();
+    while (node && !node->isElementNode())
+        node = node->previousSibling();
+    return toElement(node);
+}
+
 inline Element* nextSibling(const Node& current)
 {
     Node* node = current.nextSibling();
     while (node && !node->isElementNode())
         node = node->nextSibling();
     return toElement(node);
 }
 
 }
 
 }
 
 #endif