Add toHTMLSummaryElement(), and use it.

Source/core/html/HTMLSummaryElement.h

 /*
  * Copyright (C) 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 26 matching lines @@
     HTMLSummaryElement(const QualifiedName&, Document&);
 
     virtual RenderObject* createRenderer(RenderStyle*);
     virtual void defaultEventHandler(Event*);
     virtual void didAddUserAgentShadowRoot(ShadowRoot*) OVERRIDE;
     HTMLDetailsElement* detailsElement() const;
 
     bool supportsFocus() const OVERRIDE;
 };
 
+inline HTMLSummaryElement* toHTMLSummaryElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::summaryTag));
+    return static_cast<HTMLSummaryElement*>(node);
+}
+
 }
 
 #endif // HTMLSummaryElement_h