[bindings] Make all CustomEvent.initCustomEvent arguments non-optional and remove custom binding.

Source/core/events/CustomEvent.idl

 /*
  * Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies)
  *
  * 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 @@
  */
 
 // https://dom.spec.whatwg.org/#interface-customevent
 
 [
     Constructor(DOMString type, optional CustomEventInit eventInitDict),
     Exposed=(Window,Worker),
 ] interface CustomEvent : Event {
     [Custom=Getter] readonly attribute any detail;
 
-    // FIXME: initCustomEvent()'s arguments should not be optional.
-    [Custom, Measure] void initCustomEvent([Default=Undefined] optional DOMString type,
-                                           [Default=Undefined] optional boolean bubbles,
-                                           [Default=Undefined] optional boolean cancelable,
-                                           [Default=Undefined] optional any detail);
+    [Measure] void initCustomEvent(DOMString type, boolean bubbles, boolean cancelable, any detail);
 };