Make DataTransferItemList.add(data, type) arguments mandatory

Source/core/dom/DataTransferItemList.idl

 /*
  * 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 18 matching lines @@
  */
 
 [
     NoInterfaceObject
 ] interface DataTransferItemList {
     readonly attribute long length;
     getter DataTransferItem item([Default=Undefined] optional unsigned long index);
 
     void clear();
     void add(File? file);
-    [RaisesException] void add([Default=Undefined] optional DOMString data,
-             [Default=Undefined] optional DOMString type);
+    [RaisesException] void add(DOMString data, DOMString type);
 };