Make UserActivityDetector a singleton

chrome/browser/idle_chromeos.cc

 // 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.
 
 #include "chrome/browser/idle.h"
 
 #include "ash/session/session_state_delegate.h"
 #include "ash/shell.h"
 #include "base/time/time.h"
 #include "ui/wm/core/user_activity_detector.h"
 
 void CalculateIdleTime(IdleTimeCallback notify) {
-#if defined(USE_ATHENA)
-  // TODO(oshima): Implement this for athena. crbug.com/408752
-  base::TimeDelta idle_time;
-  NOTIMPLEMENTED();
-#else
   base::TimeDelta idle_time = base::TimeTicks::Now() -
-      ash::Shell::GetInstance()->user_activity_detector()->last_activity_time();
-#endif
+      wm::UserActivityDetector::Get()->last_activity_time();
   notify.Run(static_cast<int>(idle_time.InSeconds()));
 }
 
 bool CheckIdleStateIsLocked() {
 #if defined(USE_ATHENA)
   return false;
 #else
   return ash::Shell::GetInstance()->session_state_delegate()->IsScreenLocked();
 #endif
 }