Make UserActivityDetector a singleton

ui/wm/core/user_activity_detector.h

Index: ui/wm/core/user_activity_detector.h
diff --git a/ui/wm/core/user_activity_detector.h b/ui/wm/core/user_activity_detector.h
index 0b92bc3b7c38c9888451a5675f33d56debfc909b..13e290e5104e28df8757f76342c75bd20090e154 100644
--- a/ui/wm/core/user_activity_detector.h
+++ b/ui/wm/core/user_activity_detector.h
@@ -29,6 +29,9 @@ class WM_EXPORT UserActivityDetector : public ui::EventHandler {
   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; }