Oilpan: move FetchBodyStream to the heap.

Source/modules/serviceworkers/FetchBodyStream.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.
 
 // http://fetch.spec.whatwg.org/#fetchbodystream
 [
     RuntimeEnabled=ServiceWorker,
     Exposed=ServiceWorker,
     NoInterfaceObject,
-    ActiveDOMObject
+    ActiveDOMObject,
+    WillBeGarbageCollected,
 ] interface FetchBodyStream {
     [CallWith=ScriptState] Promise asArrayBuffer();
     [CallWith=ScriptState] Promise asBlob();
     [CallWith=ScriptState] Promise asJSON();
     [CallWith=ScriptState] Promise asText();
 
     // Still to be implemented.
     // [CallWith=ScriptState] Promise asFormData();
 };