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

Source/core/html/HTMLDetailsElement.cpp

 /*
  * Copyright (C) 2010, 2011 Nokia Corporation and/or its subsidiary(-ies)
  *
  * 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
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 65 matching lines @@
     content->setInlineStyleProperty(CSSPropertyDisplay, CSSValueNone);
     root.appendChild(content.release());
 }
 
 Element* HTMLDetailsElement::findMainSummary() const
 {
     if (HTMLSummaryElement* summary = Traversal<HTMLSummaryElement>::firstChild(*this))
         return summary;
 
     HTMLContentElement* content = toHTMLContentElement(userAgentShadowRoot()->firstChild());
-    ASSERT(content->firstChild() && content->firstChild()->hasTagName(summaryTag));
+    ASSERT(content->firstChild() && isHTMLSummaryElement(*content->firstChild()));
     return toElement(content->firstChild());
 }
 
 void HTMLDetailsElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
 {
     if (name == openAttr) {
         bool oldValue = m_isOpen;
         m_isOpen = !value.isNull();
         if (m_isOpen == oldValue)
             return;
@@ skipping 18 matching lines @@
 {
     setAttribute(openAttr, m_isOpen ? nullAtom : emptyAtom);
 }
 
 bool HTMLDetailsElement::isInteractiveContent() const
 {
     return true;
 }
 
 }