Make UserActivityDetector a singleton

ash/system/chromeos/power/power_event_observer.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/system/chromeos/power/power_event_observer.h"
 
 #include "ash/session/session_state_delegate.h"
 #include "ash/shell.h"
 #include "ash/system/tray/system_tray_notifier.h"
 #include "ash/wm/power_button_controller.h"
@@ skipping 32 matching lines @@
   // suspend and ask the session manager to lock the screen.
   if (!screen_locked_ && delegate->ShouldLockScreenBeforeSuspending() &&
       delegate->CanLockScreen()) {
     screen_lock_callback_ = chromeos::DBusThreadManager::Get()->
         GetPowerManagerClient()->GetSuspendReadinessCallback();
     VLOG(1) << "Requesting screen lock from PowerEventObserver";
     chromeos::DBusThreadManager::Get()->GetSessionManagerClient()->
         RequestLockScreen();
   }
 
-  shell->user_activity_detector()->OnDisplayPowerChanging();
+  wm::UserActivityDetector::Get()->OnDisplayPowerChanging();
   shell->display_configurator()->SuspendDisplays();
 }
 
 void PowerEventObserver::SuspendDone(const base::TimeDelta& sleep_duration) {
   Shell::GetInstance()->display_configurator()->ResumeDisplays();
   Shell::GetInstance()->system_tray_notifier()->NotifyRefreshClock();
 }
 
 void PowerEventObserver::ScreenIsLocked() {
   screen_locked_ = true;
 
   // Stop blocking suspend after the screen is locked.
   if (!screen_lock_callback_.is_null()) {
     VLOG(1) << "Screen locked due to suspend";
     // Run the callback asynchronously.  ScreenIsLocked() is currently
     // called asynchronously after RequestLockScreen(), but this guards
     // against it being made synchronous later.
     base::MessageLoop::current()->PostTask(FROM_HERE, screen_lock_callback_);
     screen_lock_callback_.Reset();
   } else {
     VLOG(1) << "Screen locked without suspend";
   }
 }
 
 void PowerEventObserver::ScreenIsUnlocked() {
   screen_locked_ = false;
 }
 
 }  // namespace ash