Add toHTMLDialogElement(), and use it.

Source/core/html/HTMLDialogElement.h

 /*
  * Copyright (C) 2012 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:
  *
  * 1.  Redistributions of source code must retain the above copyright
  *     notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
@@ skipping 43 matching lines @@
     virtual bool shouldBeReparentedUnderRenderView(const RenderStyle*) const OVERRIDE;
 
     void closeDialog(const String& returnValue = String());
     void reposition();
 
     bool m_topIsValid;
     LayoutUnit m_top;
     String m_returnValue;
 };
 
+inline HTMLDialogElement* toHTMLDialogElement(Node* node)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(!node || node->hasTagName(HTMLNames::dialogTag));
+    return static_cast<HTMLDialogElement*>(node);
+}
+
 } // namespace WebCore
 
 #endif