Implement mediaGalleriesPrivate api to notify extensions about gallery changed events.

chrome/common/extensions/api/media_galleries_private.idl

 // 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.
 
 // This is a private API for M23. This will be superceded by the
 // systeminfo.storage API in M24.
 
 namespace mediaGalleriesPrivate {
     // A dictionary that describes an attached device.
   [inline_doc] dictionary DeviceAttachmentDetails {
     // The name of the device.
     DOMString deviceName;
 
     // A transient id that unique identifies the device.
     DOMString deviceId;
   };
 
   // A dictionary that describes a detached device.
   [inline_doc] dictionary DeviceDetachmentDetails {
     // A transient id that unique identifies the device.
     DOMString deviceId;
   };
 
+  // A dictionary that describes the modified gallery.
+  [inline_doc] dictionary GalleryChangeDetails {
+    // Gallery identifier.
+    long galleryId;
+  };
+
   interface Events {
     // Fired when a media device gets attached.
     static void onDeviceAttached(DeviceAttachmentDetails details);
 
     // Fired when a media device gets detached.
     static void onDeviceDetached(DeviceDetachmentDetails details);
+
+    // Fired when a media gallery is changed.
+    static void onGalleryChanged(GalleryChangeDetails details);
+  };
+
+  // A dictionary that describes the add gallery watch request results.
+  dictionary AddGalleryWatchResult {
+    long galleryId;
+    boolean success;
+  };
+
+  callback AddGalleryWatchCallback = void (AddGalleryWatchResult result);
+
+  interface Functions {
+    static void addGalleryWatch(long galleryId,
+                                AddGalleryWatchCallback callback);
+    static void removeGalleryWatch(long galleryId);
   };
 };