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

third_party/WebKit/Source/core/events/Event.idl

 /*
  * Copyright (C) 2006, 2007, 2009, 2011 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-event
 
 [
     Constructor(DOMString type, optional EventInit eventInitDict),
     Exposed=(Window,Worker),
-    GarbageCollected,
 ] interface Event {
     readonly attribute DOMString type;
     readonly attribute EventTarget? target;
     readonly attribute EventTarget? currentTarget;
 
     const unsigned short NONE = 0;
     const unsigned short CAPTURING_PHASE = 1;
     const unsigned short AT_TARGET = 2;
     const unsigned short BUBBLING_PHASE = 3;
     readonly attribute unsigned short eventPhase;
@@ skipping 46 matching lines @@
     const unsigned short KEYPRESS  = 1024;
     const unsigned short DRAGDROP  = 2048;
     const unsigned short FOCUS     = 4096;
     const unsigned short BLUR      = 8192;
     const unsigned short SELECT    = 16384;
     const unsigned short CHANGE    = 32768;
     [MeasureAs=EventSrcElement] readonly attribute EventTarget srcElement;
     [MeasureAs=EventReturnValue, CallWith=ExecutionContext, ImplementedAs=legacyReturnValue] attribute boolean returnValue;
     [MeasureAs=EventCancelBubble] attribute boolean cancelBubble;
 };