chromeos: Introduce SetClient() on ash::mojom::SystemTray 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 client interface.
+  SetClient(SystemTrayClient client);
+
   // Sets the clock to use 24 hour time formatting if |use_24_hour| is true.
   // Otherwise sets 12 hour time formatting.
   SetUse24HourClock(bool use_24_hour);
 };
 
 // Allows ash system tray to control a client (e.g. Chrome browser). Requests
 // often involve preferences or web UI that is not available to ash.
 interface SystemTrayClient {
   // Shows general settings UI.
   ShowSettings();
@@ skipping 58 matching lines @@
   // 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();
 };