Make UserActivityDetector a singleton

ui/wm/core/user_activity_detector.h

 // 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.
 
 #ifndef UI_WM_CORE_USER_ACTIVITY_DETECTOR_H_
 #define UI_WM_CORE_USER_ACTIVITY_DETECTOR_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/observer_list.h"
@@ skipping 11 matching lines @@
   // Minimum amount of time between notifications to observers.
   static const int kNotifyIntervalMs;
 
   // Amount of time that mouse events should be ignored after notification
   // is received that displays' power states are being changed.
   static const int kDisplayPowerChangeIgnoreMouseMs;
 
   UserActivityDetector();
   ~UserActivityDetector() override;
 
+  // Returns the UserActivityDetector instance if one was created.
+  static UserActivityDetector* Get();
+
   base::TimeTicks last_activity_time() const { return last_activity_time_; }
 
   void set_now_for_test(base::TimeTicks now) { now_for_test_ = now; }
 
   bool HasObserver(UserActivityObserver* observer) const;
   void AddObserver(UserActivityObserver* observer);
   void RemoveObserver(UserActivityObserver* observer);
 
   // Called when displays are about to be turned on or off.
   void OnDisplayPowerChanging();
@@ skipping 30 matching lines @@
   // is to avoid reporting mouse events that occur when displays are turned
   // on or off as user activity.
   base::TimeTicks honor_mouse_events_time_;
 
   DISALLOW_COPY_AND_ASSIGN(UserActivityDetector);
 };
 
 }  // namespace wm
 
 #endif  // UI_WM_CORE_USER_ACTIVITY_DETECTOR_H_