Use new is*Element() helper functions further more in HTML code

Source/core/html/HTMLTableCellElement.cpp

 /*
  * Copyright (C) 1997 Martin Jones (mjones@kde.org)
  *           (C) 1997 Torben Weis (weis@kde.org)
  *           (C) 1998 Waldo Bastian (bastian@kde.org)
  *           (C) 1999 Lars Knoll (knoll@kde.org)
  *           (C) 1999 Antti Koivisto (koivisto@kde.org)
  * Copyright (C) 2003, 2004, 2005, 2006, 2010 Apple Inc. 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
@@ skipping 49 matching lines @@
 }
 
 int HTMLTableCellElement::rowSpan() const
 {
     const AtomicString& rowSpanValue = fastGetAttribute(rowspanAttr);
     return max(1, min(rowSpanValue.toInt(), maxRowspan));
 }
 
 int HTMLTableCellElement::cellIndex() const
 {
-    int index = 0;
-    if (!parentElement() || !parentElement()->hasTagName(trTag))
+    if (!isHTMLTableRowElement(parentElement()))
         return -1;
 
-    for (const Node * node = previousSibling(); node; node = node->previousSibling()) {
-        if (node->hasTagName(tdTag) || node->hasTagName(thTag))
-            index++;
-    }
+    int index = 0;
+    for (const HTMLTableCellElement* element = Traversal<HTMLTableCellElement>::previousSibling(*this); element; element = Traversal<HTMLTableCellElement>::previousSibling(*element))
+        ++index;
 
     return index;
 }
 
 bool HTMLTableCellElement::isPresentationAttribute(const QualifiedName& name) const
 {
     if (name == nowrapAttr || name == widthAttr || name == heightAttr)
         return true;
     return HTMLTablePartElement::isPresentationAttribute(name);
 }
@@ skipping 82 matching lines @@
 
     RenderTableCell* tableCellRenderer = toRenderTableCell(cellRenderer);
     RenderTableCell* cellAboveRenderer = tableCellRenderer->table()->cellAbove(tableCellRenderer);
     if (!cellAboveRenderer)
         return 0;
 
     return toHTMLTableCellElement(cellAboveRenderer->node());
 }
 
 } // namespace WebCore