Implement author-constructible ReadableStream using V8 extras

third_party/WebKit/Source/core/streams/ReadableStream.idl

 // Copyright 2014 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 type is marked NoInterfaceObject while we work to replace it with the
+// V8 extras implementation. It is still used for now to implement fetch body
+// streams, and so we can't remove it quite yet.
+
 [
     GarbageCollected,
     Exposed=(Window,Worker),
+    NoInterfaceObject
 ] interface ReadableStream {
     [CallWith=ExecutionContext, RaisesException] ReadableStreamReader getReader();
     [CallWith=ScriptState] Promise<void> cancel(optional any reason);
 };