Make UserActivityDetector a singleton

ash/shell.h

Index: ash/shell.h
diff --git a/ash/shell.h b/ash/shell.h
index 51c912b6b36e3a457a0b256514999265ad5b718a..a4ef6a2b7aaade0a00232919a96adcfd921dc843 100644
--- a/ash/shell.h
+++ b/ash/shell.h
@@ -68,7 +68,6 @@ class InputMethodEventFilter;
 class NestedAcceleratorController;
 class ShadowController;
 class VisibilityController;
-class UserActivityDetector;
 class WindowModalityController;
 }
 
@@ -139,7 +138,6 @@ class SystemTrayNotifier;
 class ToplevelWindowEventHandler;
 class TouchTransformerController;
 class TouchObserverHUD;
-class UserActivityDetector;
 class UserWallpaperDelegate;
 class VirtualKeyboardController;
 class VideoActivityNotifier;
@@ -368,9 +366,6 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   MruWindowTracker* mru_window_tracker() {
     return mru_window_tracker_.get();
   }
-  ::wm::UserActivityDetector* user_activity_detector() {
-    return user_activity_detector_.get();
-  }
   VideoDetector* video_detector() {
     return video_detector_.get();
   }
@@ -661,7 +656,6 @@ class ASH_EXPORT Shell : public SystemModalContainerEventFilterDelegate,
   scoped_ptr<PowerButtonController> power_button_controller_;
   scoped_ptr<LockStateController> lock_state_controller_;
   scoped_ptr<MruWindowTracker> mru_window_tracker_;
-  scoped_ptr< ::wm::UserActivityDetector> user_activity_detector_;
   scoped_ptr<VideoDetector> video_detector_;
   scoped_ptr<WindowCycleController> window_cycle_controller_;
   scoped_ptr<WindowSelectorController> window_selector_controller_;