Oilpan: keep StashedPortCollection always on the heap.

Source/modules/serviceworkers/StashedPortCollection.idl

 // Copyright 2015 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.
 
 // https://github.com/mkruisselbrink/navigator-connect/blob/gh-pages/docs/api-idea.md#persisted-messageports
 // TODO(mek): Link to actual spec when this is added there.
 [
     Exposed=ServiceWorker,
+    GarbageCollected,
     RuntimeEnabled=NavigatorConnect,
     TypeChecking=Interface,
 ] interface StashedPortCollection : EventTarget {
     [CallWith=ScriptState] StashedMessagePort add(USVString name, MessagePort port);
     // Promise<sequence<StashedMessagePort>> match(UVString name);
     attribute EventHandler onmessage;
 };