[Oilpan] Apply RefCountedGarbageCollectedEventTarget on AbstractWorker

Source/core/workers/SharedWorker.idl

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  * Copyright (C) 2010 Apple 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.
@@ skipping 18 matching lines @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
 // https://html.spec.whatwg.org/#shared-workers-and-the-sharedworker-interface
 
 [
     ActiveDOMObject,
     // TODO(philipj): The name argument should not have a default null value.
     Constructor(DOMString scriptURL, optional DOMString name = null),
     ConstructorCallWith=ExecutionContext,
+    GarbageCollected,
     // TODO(philipj): Exposed=(Window,Worker),
     RaisesException=Constructor,
     RuntimeEnabled=SharedWorker,
 ] interface SharedWorker : EventTarget {
     readonly attribute MessagePort port;
 };
 
 SharedWorker implements AbstractWorker;