Use Mojo Permission Service when requesting Notifications permission.

content/common/permission_service.mojom

 // 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.
 
 module content;
 
 enum PermissionStatus {
   GRANTED,
   DENIED,
   ASK
 };
 
 enum PermissionName {
   GEOLOCATION,
   MIDI_SYSEX,
+  NOTIFICATIONS,
 };
 
 // The Permission service provides permission handling capabilities by exposing
 // methods to check, request, and revoke permissions. It also allows a client to
 // start listening to permission changes.
 interface PermissionService {
   HasPermission(PermissionName permission, string origin)
     => (PermissionStatus status);
   RequestPermission(PermissionName permission, string origin)
     => (PermissionStatus status);
 };