Make IDL Callbacks non-refcounted by default

Source/core/dom/DataTransferItem.h

 /*
  * 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 @@
 public:
     static PassRefPtr<DataTransferItem> create(PassRefPtr<Clipboard>, PassRefPtr<ChromiumDataObjectItem>);
     ~DataTransferItem();
 
     static const char kindString[];
     static const char kindFile[];
 
     String kind() const;
     String type() const;
 
-    void getAsString(ExecutionContext*, PassRefPtr<StringCallback>) const;
+    void getAsString(ExecutionContext*, PassOwnPtr<StringCallback>) const;
     PassRefPtr<Blob> getAsFile() const;
 
     Clipboard* clipboard() { return m_clipboard.get(); }
     ChromiumDataObjectItem* dataObjectItem() { return m_item.get(); }
 
 private:
     DataTransferItem(PassRefPtr<Clipboard>, PassRefPtr<ChromiumDataObjectItem>);
 
     RefPtr<Clipboard> m_clipboard;
     RefPtr<ChromiumDataObjectItem> m_item;
 };
 
 } // namespace WebCore
 
 #endif // DataTransferItem_h