Implement PermissionService::GetNextPermissionChange.

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;
 
 import "content/public/common/permission_status.mojom";
 
 enum PermissionName {
   GEOLOCATION,
   NOTIFICATIONS,
   PUSH_NOTIFICATIONS,
   MIDI_SYSEX,
   PROTECTED_MEDIA_IDENTIFIER,
 };
 
 // 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, bool user_gesture)
     => (PermissionStatus status);
   RevokePermission(PermissionName permission, string origin)
     => (PermissionStatus status);
+
+  // Runs the callback next time there is a permission status change for the
+  // given { permission, origin }. Callers of this method will have to call it
+  // again if they want to keep listening to the changes. To prevent race
+  // conditions, the caller must pass the last known value.
+  GetNextPermissionChange(PermissionName permission,
+                          string origin,
+                          PermissionStatus last_known_status)
+    => (PermissionStatus status);
 };