chromeos: Convert system tray session length limit to mojo

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 467 matching lines @@
     "system/screen_security/screen_tray_item.cc",
     "system/screen_security/screen_tray_item.h",
     "system/session/last_window_closed_observer.h",
     "system/session/logout_button_observer.h",
     "system/session/logout_button_tray.cc",
     "system/session/logout_button_tray.h",
     "system/session/logout_confirmation_controller.cc",
     "system/session/logout_confirmation_controller.h",
     "system/session/logout_confirmation_dialog.cc",
     "system/session/logout_confirmation_dialog.h",
-    "system/session/session_length_limit_observer.h",
     "system/session/tray_session_length_limit.cc",
     "system/session/tray_session_length_limit.h",
     "system/status_area_focus_observer.h",
     "system/status_area_layout_manager.cc",
     "system/status_area_layout_manager.h",
     "system/status_area_widget.cc",
     "system/status_area_widget.h",
     "system/status_area_widget_delegate.cc",
     "system/status_area_widget_delegate.h",
     "system/supervised/tray_supervised_user.cc",
@@ skipping 1024 matching lines @@
     "//content/public/app:both",
     "//device/bluetooth",
   ]
 
   data_deps = [
     "//ash/strings:ash_test_strings",
     "//ash/resources:ash_test_resources_with_content_100_percent",
     "//ash/resources:ash_test_resources_200_percent",
   ]
 }