Implement Permission change observing in Blink.

Source/modules/permissions/PermissionStatus.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.
 
 // Specification at:
 // https://w3c.github.io/permissions/
 
 enum PermissionState {
     "granted",
     "denied",
     "prompt"
 };
 
 [
+    ActiveDOMObject,
+    GarbageCollected,
     Exposed=(Window,Worker),
     RuntimeEnabled=Permissions,
 ] interface PermissionStatus : EventTarget {
     readonly attribute PermissionState status;
              attribute EventHandler onchange;
 };