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

third_party/WebKit/Source/modules/offscreencanvas2d/OffscreenCanvasRenderingContext2D.idl

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Note: The official spec is WIP at wiki.whatwg.org/wiki/OffscreenCanvas.
 
 [
-    GarbageCollected,
     Exposed=(Window,Worker),
     RuntimeEnabled=ExperimentalCanvasFeatures,
 ] interface OffscreenCanvasRenderingContext2D {
     // back-reference to the canvas
     [ImplementedAs=getOffscreenCanvas] readonly attribute OffscreenCanvas offscreenCanvas;
 
     // colors and styles
     attribute (DOMString or CanvasGradient or CanvasPattern) strokeStyle; // (default black)
     attribute (DOMString or CanvasGradient or CanvasPattern) fillStyle; // (default black)
 
@@ skipping 33 matching lines @@
     void resetTransform();
 
     // shadows
     attribute unrestricted double shadowOffsetX;
     attribute unrestricted double shadowOffsetY;
     attribute unrestricted double shadowBlur;
     attribute DOMString shadowColor;
 };
 
 OffscreenCanvasRenderingContext2D implements CanvasPathMethods;