Have bindings layer assume and insist that all interface types are GCed.

third_party/WebKit/Source/core/xmlhttprequest/XMLHttpRequestEventTarget.idl

 /*
  * Copyright (C) 2008, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2013 Samsung Electronics. 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
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
  *
  * THIS SOFTWARE IS PROVIDED BY APPLE COMPUTER, INC. ``AS IS'' AND ANY
  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE COMPUTER, INC. OR
  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
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  */
 
 // https://xhr.spec.whatwg.org/#xmlhttprequesteventtarget
 
 [
-    GarbageCollected,
     Exposed=(Window,DedicatedWorker,SharedWorker)
 ] interface XMLHttpRequestEventTarget : EventTarget {
     // event handlers
     attribute EventHandler onloadstart;
     attribute EventHandler onprogress;
     attribute EventHandler onabort;
     attribute EventHandler onerror;
     attribute EventHandler onload;
     attribute EventHandler ontimeout;
     attribute EventHandler onloadend;
 };