Remove redundant [WillBeGarbageCollected] uses on derived EventTargets.

Source/modules/serviceworkers/ServiceWorker.idl

 /*
  * Copyright (C) 2013 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 23 matching lines @@
     "installing",
     "installed",
     "activating",
     "activated",
     "redundant"
 };
 
 [
     ActiveDOMObject,
     RuntimeEnabled=ServiceWorker,
-    WillBeGarbageCollected
 ] interface ServiceWorker : EventTarget {
 
     // FIXME: Should inherit this from Worker.
     [Custom, RaisesException] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
 
     [RaisesException] void terminate();
 
     readonly attribute ScalarValueString scriptURL;
     readonly attribute ServiceWorkerState  state;
 
     attribute EventHandler onstatechange;
 };
 
 ServiceWorker implements AbstractWorker;