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

third_party/WebKit/Source/core/streams/UnderlyingSourceBase.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.
 
 // This is not a spec interface. Instead, it is used to generate bindings
 // so that Blink code which wishes to create a stream can derive from
 // UnderlyingSourceBase, and a JavaScript object can then be generated
 // automatically for use in initializing a ReadableStream.
 
 [
     ActiveScriptWrappable,
     DependentLifetime,
-    GarbageCollected,
     NoInterfaceObject,
 ]
 interface UnderlyingSourceBase {
     [CallWith=ScriptState, ImplementedAs=startWrapper] Promise<void> start(any stream);
     [CallWith=ScriptState] Promise<void> pull();
     [CallWith=ScriptState, ImplementedAs=cancelWrapper] Promise<void> cancel([Default=Undefined] optional any reason);
 };