IDL: make optional arguments (without default) explicit sometimes

Source/core/frame/History.idl

 /*
  * Copyright (C) 2007, 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
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 13 matching lines @@
  */
 
 [
     WillBeGarbageCollected,
 ] interface History {
     readonly attribute unsigned long length;
     [Custom=Getter] readonly attribute SerializedScriptValue state;
 
     [CallWith=ExecutionContext] void back();
     [CallWith=ExecutionContext] void forward();
-    [CallWith=ExecutionContext] void go([Default=Undefined] optional long distance);
+    [CallWith=ExecutionContext] void go(optional long distance);
 
     [Custom, PerContextEnabled=PushState, RaisesException] void pushState(any data, DOMString title, optional DOMString url);
     [Custom, PerContextEnabled=PushState, RaisesException] void replaceState(any data, DOMString title, optional DOMString url);
 };