IDL: make optional arguments (without default) explicit sometimes

Source/modules/gamepad/GamepadList.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:
  *
  * 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 10 matching lines @@
  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
  * DAMAGE.
  */
 
 [
     NoInterfaceObject,
     GarbageCollected,
 ] interface GamepadList {
     readonly attribute unsigned long length;
-    getter Gamepad item([Default=Undefined] optional unsigned long index);
+    getter Gamepad item(optional unsigned long index);
 };