Remove redundant [WillBeGarbageCollected] uses on derived EventTargets.

Source/core/workers/Worker.idl

 /*
  * Copyright (C) 2008, 2010 Apple Inc. All Rights Reserved.
  * Copyright (C) 2011 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:
  * 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
@@ skipping 14 matching lines @@
  *
  */
 
 // http://www.whatwg.org/specs/web-apps/current-work/#dedicated-workers-and-the-worker-interface
 
 [
     ActiveDOMObject,
     Constructor(DOMString scriptUrl),
     ConstructorCallWith=ExecutionContext,
     RaisesException=Constructor,
-    WillBeGarbageCollected,
 ] interface Worker : EventTarget {
 
     attribute EventHandler onmessage;
 
     [Custom, RaisesException] void postMessage(SerializedScriptValue message, optional sequence<Transferable> transfer);
     void terminate();
 };
 
 Worker implements AbstractWorker;