Make UserActivityDetector a singleton

ash/shell.cc

Index: ash/shell.cc
diff --git a/ash/shell.cc b/ash/shell.cc
index 9de2e1581e2b3cb1c284339fabaef84059a639fa..ed5ac238cac04e2d3cd0395e0ca07ecbbeaec0c5 100644
--- a/ash/shell.cc
+++ b/ash/shell.cc
@@ -681,7 +681,11 @@ Shell::~Shell() {
   RemovePreTargetHandler(speech_feedback_handler_.get());
   speech_feedback_handler_.reset();
 #endif
-  RemovePreTargetHandler(user_activity_detector_.get());
+
+#if defined(OS_CHROMEOS)
+  RemovePreTargetHandler(::wm::UserActivityDetector::Get());
+#endif
+
   RemovePreTargetHandler(overlay_filter_.get());
   RemovePreTargetHandler(input_method_filter_.get());
   RemovePreTargetHandler(accelerator_filter_.get());
@@ -912,10 +916,11 @@ void Shell::Init(const ShellInitParams& init_params) {
 
   // The order in which event filters are added is significant.
 
+#if defined(OS_CHROMEOS)
   // wm::UserActivityDetector passes events to observers, so let them get
   // rewritten first.
-  user_activity_detector_.reset(new ::wm::UserActivityDetector);
-  AddPreTargetHandler(user_activity_detector_.get());
+  AddPreTargetHandler(::wm::UserActivityDetector::Get());
+#endif
 
   overlay_filter_.reset(new OverlayEventFilter);
   AddPreTargetHandler(overlay_filter_.get());
@@ -1054,8 +1059,8 @@ void Shell::Init(const ShellInitParams& init_params) {
           new ash::system::BrightnessControllerChromeos).Pass());
 
   power_event_observer_.reset(new PowerEventObserver());
-  user_activity_notifier_.reset(
-      new ui::UserActivityPowerManagerNotifier(user_activity_detector_.get()));
+  user_activity_notifier_.reset(new ui::UserActivityPowerManagerNotifier(
+      ::wm::UserActivityDetector::Get()));
   video_activity_notifier_.reset(
       new VideoActivityNotifier(video_detector_.get()));
   bluetooth_notification_controller_.reset(new BluetoothNotificationController);