Add UseCounters for init*Event methods that aren't already measured

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 14 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] void initCustomEvent([Default=Undefined] optional DOMString typeArg,
-                                  [Default=Undefined] optional boolean canBubbleArg,
-                                  [Default=Undefined] optional boolean cancelableArg,
-                                  [Default=Undefined] optional any detailArg);
+    [Custom, Measure] void initCustomEvent([Default=Undefined] optional DOMString type,
+                                           [Default=Undefined] optional boolean bubbles,
+                                           [Default=Undefined] optional boolean cancelable,
+                                           [Default=Undefined] optional any detail);
 };