Add toHTMLSummaryElement(), and use it.

Source/core/html/shadow/DetailsMarkerControl.cpp

 /*
  * Copyright (C) 2011 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 38 matching lines @@
     return new RenderDetailsMarker(this);
 }
 
 bool DetailsMarkerControl::rendererIsNeeded(const NodeRenderingContext& context)
 {
     return summaryElement()->isMainSummary() && HTMLDivElement::rendererIsNeeded(context);
 }
 
 HTMLSummaryElement* DetailsMarkerControl::summaryElement()
 {
-    Element* element = shadowHost();
-    ASSERT_WITH_SECURITY_IMPLICATION(!element || element->hasTagName(summaryTag));
-    return static_cast<HTMLSummaryElement*>(element);
+    return toHTMLSummaryElement(shadowHost());
 }
 
 }