Make UserActivityDetector a singleton

ash/display/display_change_observer_chromeos.cc

 // Copyright 2013 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.
 
 #include "ash/display/display_change_observer_chromeos.h"
 
 #include <algorithm>
 #include <map>
 #include <set>
 #include <string>
@@ skipping 249 matching lines @@
   }
 
   AssociateTouchscreens(
       &displays, ui::DeviceDataManager::GetInstance()->touchscreen_devices());
   // DisplayManager can be null during the boot.
   Shell::GetInstance()->display_manager()->OnNativeDisplaysChanged(displays);
 
   // For the purposes of user activity detection, ignore synthetic mouse events
   // that are triggered by screen resizes: http://crbug.com/360634
   ::wm::UserActivityDetector* user_activity_detector =
-      Shell::GetInstance()->user_activity_detector();
+      ::wm::UserActivityDetector::Get();
   if (user_activity_detector)
     user_activity_detector->OnDisplayPowerChanging();
 }
 
 void DisplayChangeObserver::OnAppTerminating() {
 #if defined(USE_ASH)
   // Stop handling display configuration events once the shutdown
   // process starts. crbug.com/177014.
   Shell::GetInstance()->display_configurator()->PrepareForExit();
 #endif
@@ skipping 10 matching lines @@
 
 void DisplayChangeObserver::OnTouchscreenDeviceConfigurationChanged() {
   OnDisplayModeChanged(
       Shell::GetInstance()->display_configurator()->cached_displays());
 }
 
 void DisplayChangeObserver::OnKeyboardDeviceConfigurationChanged() {
 }
 
 }  // namespace ash