mash: Add SystemTrayClient interface, use to show date settings

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.
   SetUse24HourClock(bool use_24_hour);
 };
 
-// TODO(jamescook): SystemTrayDelegate to communicate from ash to chrome.
+// 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 the settings related to date, timezone etc.
+  ShowDateSettings();
+};