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

third_party/WebKit/Source/core/dom/DOMImplementation.idl

 /*
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2006 Samuel Weinig <sam.weinig@gmail.com>
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
  *
  * This library is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  * Library General Public License for more details.
  *
  * You should have received a copy of the GNU Library General Public License
  * along with this library; see the file COPYING.LIB.  If not, write to
  * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
  * Boston, MA 02110-1301, USA.
  */
 
 // https://dom.spec.whatwg.org/#interface-domimplementation
 
 [
     SetWrapperReferenceFrom=document,
-    GarbageCollected,
 ] interface DOMImplementation {
     [NewObject, RaisesException] DocumentType createDocumentType(DOMString qualifiedName, DOMString publicId, DOMString systemId);
     [NewObject, RaisesException] XMLDocument createDocument(DOMString? namespaceURI, [TreatNullAs=EmptyString] DOMString qualifiedName, optional DocumentType? doctype = null);
     // FIXME: createHTMLDocument should return a Document. crbug.com/238368
     // FIXME: The title argument should not have a default value. crbug.com/335871
     [NewObject] HTMLDocument createHTMLDocument(optional DOMString title = null);
 
     // useless; always returns true
     boolean hasFeature();
 };