Use toDocument instead of static_cast<Document*>.

Source/WebKit/chromium/src/WebDocument.cpp

 /*
  * Copyright (C) 2009 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 270 matching lines @@
 }
 
 WebDocument& WebDocument::operator=(const PassRefPtr<Document>& elem)
 {
     m_private = elem;
     return *this;
 }
 
 WebDocument::operator PassRefPtr<Document>() const
 {
-    return static_cast<Document*>(m_private.get());
+    return toDocument(m_private.get());
 }
 
 } // namespace WebKit