Get rid of custom code for clipboard.clearData()

Source/core/dom/Clipboard.idl

 /*
  * Copyright (C) 2008 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
@@ skipping 15 matching lines @@
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 [
 ] interface Clipboard {
              [TreatReturnedNullStringAs=Undefined] attribute DOMString dropEffect;
              [TreatReturnedNullStringAs=Undefined] attribute DOMString effectAllowed;
     readonly attribute DOMString[] types;
     readonly attribute FileList files;
 
-    [Custom, RaisesException] void clearData(optional DOMString type);
+    void clearData(optional DOMString type);
     DOMString getData(DOMString type);
     boolean setData(DOMString type, DOMString data);
     [Custom, RaisesException] void setDragImage(HTMLImageElement image, long x, long y);
 
     readonly attribute DataTransferItemList items;
 };