chromeos: Migrate SignOut and RequestRestartForUpdate to mojo interface

ash/public/interfaces/system_tray.mojom

 // Copyright 2016 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 ash.mojom;
 
 // Allows clients (e.g. Chrome browser) to control the ash system tray menu.
 interface SystemTray {
   // Sets the clock to use 24 hour time formatting if |use_24_hour| is true.
   // Otherwise sets 12 hour time formatting.
@@ skipping 54 matching lines @@
   ShowNetworkCreate(string type);
 
   // Shows settings related to networking. If |network_id| is empty, shows
   // general settings. Otherwise shows settings for the individual network.
   // On devices |network_id| is a GUID, but on Linux desktop and in tests it can
   // be any string.
   ShowNetworkSettings(string network_id);
 
   // Shows UI for changing proxy settings.
   ShowProxySettings();
+
+  // Attempts to sign out the user.
+  // TODO(jamescook): Migrate to a user or login service. crbug.com/665582
+  SignOut();
+
+  // Attempts to restart the system for update.
+  RequestRestartForUpdate();
 };