Consistently use ElementTraversal API when looking for an Element

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 61 matching lines @@
 
     RefPtr<HTMLDivElement> content = HTMLDivElement::create(document());
     content->setIdAttribute(ShadowElementNames::detailsContent());
     content->appendChild(HTMLContentElement::create(document()));
     content->setInlineStyleProperty(CSSPropertyDisplay, CSSValueNone);
     root.appendChild(content.release());
 }
 
 Element* HTMLDetailsElement::findMainSummary() const
 {
-    for (Node* child = firstChild(); child; child = child->nextSibling()) {
+    for (Element* child = ElementTraversal::firstWithin(*this); child; child = ElementTraversal::nextSibling(*child)) {
         if (child->hasTagName(summaryTag))
-            return toElement(child);
+            return child;
     }
 
     HTMLContentElement* content = toHTMLContentElement(userAgentShadowRoot()->firstChild());
     ASSERT(content->firstChild() && content->firstChild()->hasTagName(summaryTag));
     return toElement(content->firstChild());
 }
 
 void HTMLDetailsElement::parseAttribute(const QualifiedName& name, const AtomicString& value)
 {
     if (name == openAttr) {
@@ skipping 22 matching lines @@
 {
     setAttribute(openAttr, m_isOpen ? nullAtom : emptyAtom);
 }
 
 bool HTMLDetailsElement::isInteractiveContent() const
 {
     return true;
 }
 
 }