IDL: make optional arguments (without default) explicit sometimes

Source/core/dom/ElementFullscreen.idl

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 partial interface Element {
     [RuntimeEnabled=FullscreenUnprefixed] void requestFullscreen();
 
     // Mozilla version
     const unsigned short ALLOW_KEYBOARD_INPUT = 1;
-    [LogActivity, LogAllWorlds, MeasureAs=PrefixedElementRequestFullScreen] void webkitRequestFullScreen([Default=Undefined] optional unsigned short flags);
+    [LogActivity, LogAllWorlds, MeasureAs=PrefixedElementRequestFullScreen] void webkitRequestFullScreen(optional unsigned short flags);
 
     // W3C version
     [LogActivity, LogAllWorlds, MeasureAs=PrefixedElementRequestFullscreen] void webkitRequestFullscreen();
 
     attribute EventHandler onwebkitfullscreenchange;
     attribute EventHandler onwebkitfullscreenerror;
 };