Avoid unchecked casts in UA shadow DOM.

third_party/WebKit/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 94 matching lines @@
   content->setInlineStyleProperty(CSSPropertyDisplay, CSSValueNone);
   root.appendChild(content);
 }
 
 Element* HTMLDetailsElement::findMainSummary() const {
   if (HTMLSummaryElement* summary =
           Traversal<HTMLSummaryElement>::firstChild(*this))
     return summary;
 
   HTMLContentElement* content =
-      toHTMLContentElement(userAgentShadowRoot()->firstChild());
+      toHTMLContentElementOrDie(userAgentShadowRoot()->firstChild());
   DCHECK(content->firstChild());
-  DCHECK(isHTMLSummaryElement(*content->firstChild()));
+  CHECK(isHTMLSummaryElement(*content->firstChild()));
   return toElement(content->firstChild());
 }
 
 void HTMLDetailsElement::parseAttribute(const QualifiedName& name,
                                         const AtomicString& oldValue,
                                         const AtomicString& value) {
   if (name == openAttr) {
     bool oldValue = m_isOpen;
     m_isOpen = !value.isNull();
     if (m_isOpen == oldValue)
@@ skipping 31 matching lines @@
 
 void HTMLDetailsElement::toggleOpen() {
   setAttribute(openAttr, m_isOpen ? nullAtom : emptyAtom);
 }
 
 bool HTMLDetailsElement::isInteractiveContent() const {
   return true;
 }
 
 }  // namespace blink