Add toPluginDocument() convenience method that takes a reference in argument

Source/core/html/PluginDocument.h

 /*
  * Copyright (C) 2006, 2008, 2009 Apple 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 55 matching lines @@
     ASSERT_WITH_SECURITY_IMPLICATION(!document || document->isPluginDocument());
     return static_cast<PluginDocument*>(document);
 }
 
 inline const PluginDocument* toPluginDocument(const Document* document)
 {
     ASSERT_WITH_SECURITY_IMPLICATION(!document || document->isPluginDocument());
     return static_cast<const PluginDocument*>(document);
 }
 
+inline PluginDocument& toPluginDocument(Document& document)
+{
+    ASSERT_WITH_SECURITY_IMPLICATION(document.isPluginDocument());
+    return static_cast<PluginDocument&>(document);
+}
+
 // This will catch anyone doing an unnecessary cast.
 void toPluginDocument(const PluginDocument*);
 
 }
 
 #endif // PluginDocument_h