ash: Use system tray mojo interface to show system update tray icon

ash/BUILD.gn

 # 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.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//testing/test.gni")
 import("//ui/base/ui_features.gni")
 
 # Historical note: Ash shipped on Windows for a couple years to support
@@ skipping 422 matching lines @@
     "common/system/tray/tray_popup_utils.h",
     "common/system/tray/tray_utils.cc",
     "common/system/tray/tray_utils.h",
     "common/system/tray/tri_view.cc",
     "common/system/tray/tri_view.h",
     "common/system/tray/view_click_listener.h",
     "common/system/tray_accessibility.cc",
     "common/system/tray_accessibility.h",
     "common/system/update/tray_update.cc",
     "common/system/update/tray_update.h",
-    "common/system/update/update_observer.h",
     "common/system/user/button_from_view.cc",
     "common/system/user/button_from_view.h",
     "common/system/user/login_status.cc",
     "common/system/user/login_status.h",
     "common/system/user/rounded_image_view.cc",
     "common/system/user/rounded_image_view.h",
     "common/system/user/tray_user.cc",
     "common/system/user/tray_user.h",
     "common/system/user/tray_user_separator.cc",
     "common/system/user/tray_user_separator.h",
@@ skipping 975 matching lines @@
     "//content/public/app:both",
     "//device/bluetooth",
   ]
 
   data_deps = [
     "//ash/common/strings:ash_test_strings",
     "//ash/resources:ash_test_resources_with_content_100_percent",
     "//ash/resources:ash_test_resources_200_percent",
   ]
 }