ash: Add a unit test for the system update tray item

ash/ash.gyp

 # Copyright (c) 2012 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.
 
 {
   'variables': {
     'chromium_code': 1,
     # These files lists are shared with the GN build.
     'ash_sources': [
       'accelerators/accelerator_commands.cc',
@@ skipping 948 matching lines @@
       'system/chromeos/session/tray_session_length_limit_unittest.cc',
       'system/chromeos/supervised/tray_supervised_user_unittest.cc',
       'system/chromeos/tray_display_unittest.cc',
       'system/date/date_view_unittest.cc',
       'system/ime/tray_ime_chromeos_unittest.cc',
       'system/overview/overview_button_tray_unittest.cc',
       'system/toast/toast_manager_unittest.cc',
       'system/tray/media_security/multi_profile_media_tray_item_unittest.cc',
       'system/tray/system_tray_unittest.cc',
       'system/tray/tray_details_view_unittest.cc',
+      'system/tray_update_unittest.cc',
       'system/user/tray_user_unittest.cc',
       'system/web_notification/ash_popup_alignment_delegate_unittest.cc',
       'system/web_notification/web_notification_tray_unittest.cc',
       'test/ash_test_helper_unittest.cc',
       'test/ash_unittests.cc',
       'tooltips/tooltip_controller_unittest.cc',
       'touch/touch_observer_hud_unittest.cc',
       'touch/touch_transformer_controller_unittest.cc',
       'touch/touchscreen_util_unittest.cc',
       'utility/screenshot_controller_unittest.cc',
@@ skipping 467 matching lines @@
                   '../tools/xdisplaycheck/xdisplaycheck.gyp:xdisplaycheck',
                 ],
               }
             ],
           ],
         },
       ],
     }],
   ],
 }