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..611bb8b1805bced740c8b5c1d6a5791ca845faa6 100644
--- a/ui/wm/core/user_activity_detector.h
+++ b/ui/wm/core/user_activity_detector.h
@@ -26,8 +26,14 @@ class WM_EXPORT UserActivityDetector : public ui::EventHandler {
   // is received that displays' power states are being changed.
   static const int kDisplayPowerChangeIgnoreMouseMs;
 
-  UserActivityDetector();
-  ~UserActivityDetector() override;
+  // Creates a UserActivityDetector instance.
+  static void Create();
+
+  // Deletes the UserActivityDetector instance.
+  static void Shutdown();

[oshima, 2014/10/31 16:53:31]

Can you keep ctor/dtor, and creation in shell. Just add static accessor only.

[pkotwicz, 2014/10/31 16:58:29]

For the sake of interest, which other classes have a static getter but not a
static constructor and destructor?

+
+  // Returns the UserActivityDetector instance if one was created.
+  static UserActivityDetector* Get();
 
   base::TimeTicks last_activity_time() const { return last_activity_time_; }
 
@@ -48,6 +54,9 @@ class WM_EXPORT UserActivityDetector : public ui::EventHandler {
   void OnGestureEvent(ui::GestureEvent* event) override;
 
  private:
+  UserActivityDetector();
+  ~UserActivityDetector() override;
+
   // Returns |now_for_test_| if set or base::TimeTicks::Now() otherwise.
   base::TimeTicks GetCurrentTime() const;