Implement mediaGalleriesPrivate api to notify extensions about gallery changed events.

chrome/common/extensions/api/_permission_features.json

 // Copyright (c) 2012 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.
 
 {
   "activeTab": {
     "channel": "dev",
     "extension_types": ["extension", "packaged_app"],
     "min_manifest_version": 2
   },
@@ skipping 199 matching lines @@
     "channel": "stable",
     "extension_types": [ "platform_app" ]
   },
   // TODO(thestig) Remove this as part of http:://crbug.com/144496
   "mediaGalleriesPrivate": {
     "channel": "stable",
     "extension_types": [ "platform_app" ],
     "whitelist": [
       "ebpbnabdhheoknfklmpddcdijjkmklkp",
       "efjnaogkjbogokcnohkmnjdojkikgobo",
-      "ejegoaikibpmikoejfephaneibodccma"
+      "ejegoaikibpmikoejfephaneibodccma",
     ]
   },
   "mediaPlayerPrivate": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"],
     "location": "component"
   },
   "metricsPrivate": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app", "platform_app"],
@@ skipping 173 matching lines @@
   },
   "webRequestBlocking": {
     "channel": "stable",
     "extension_types": ["extension", "packaged_app"]
   },
   "webview": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   }
 }