Split NonDocumentTypeChildNode from ChildNode

Source/core/dom/ChildNode.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef ChildNode_h
 #define ChildNode_h
 
-#include "core/dom/ElementTraversal.h"
 #include "core/dom/Node.h"
 
 namespace blink {
 
 class ChildNode {
 public:
-    static Element* previousElementSibling(Node& node)
-    {
-        return ElementTraversal::previousSibling(node);
-    }
-
-    static Element* nextElementSibling(Node& node)
-    {
-        return ElementTraversal::nextSibling(node);
-    }
-
     static void remove(Node& node, ExceptionState& exceptionState)
     {
         return node.remove(exceptionState);
     }
 };
 
 } // namespace blink
 
 #endif // ChildNode_h