Make UserActivityDetector a singleton

extensions/shell/browser/shell_desktop_controller.cc

Index: extensions/shell/browser/shell_desktop_controller.cc
diff --git a/extensions/shell/browser/shell_desktop_controller.cc b/extensions/shell/browser/shell_desktop_controller.cc
index f7da092386d82569459fcfb6d5684b7cb056640e..e3cf6b9de60b5e4f4b2af6fd7186668026632740 100644
--- a/extensions/shell/browser/shell_desktop_controller.cc
+++ b/extensions/shell/browser/shell_desktop_controller.cc
@@ -276,12 +276,11 @@ void ShellDesktopController::InitWindowManager() {
   cursor_manager_->SetCursor(ui::kCursorPointer);
   aura::client::SetCursorClient(host_->window(), cursor_manager_.get());
 
-  user_activity_detector_.reset(new wm::UserActivityDetector);
   host_->event_processor()->GetRootTarget()->AddPreTargetHandler(
-      user_activity_detector_.get());
+      wm::UserActivityDetector::Get());
 #if defined(OS_CHROMEOS)
-  user_activity_notifier_.reset(
-      new ui::UserActivityPowerManagerNotifier(user_activity_detector_.get()));
+  user_activity_notifier_.reset(new ui::UserActivityPowerManagerNotifier(
+      wm::UserActivityDetector::Get()));
 #endif
 }
 
@@ -323,10 +322,8 @@ void ShellDesktopController::DestroyRootWindow() {
   host_->RemoveObserver(this);
   if (input_method_filter_)
     root_window_event_filter_->RemoveHandler(input_method_filter_.get());
-  if (user_activity_detector_) {
-    host_->event_processor()->GetRootTarget()->RemovePreTargetHandler(
-        user_activity_detector_.get());
-  }
+  host_->event_processor()->GetRootTarget()->RemovePreTargetHandler(
+      wm::UserActivityDetector::Get());
   wm::FocusController* focus_controller =
       static_cast<wm::FocusController*>(focus_client_.get());
   if (focus_controller) {
@@ -341,7 +338,6 @@ void ShellDesktopController::DestroyRootWindow() {
 #if defined(OS_CHROMEOS)
   user_activity_notifier_.reset();
 #endif
-  user_activity_detector_.reset();
   host_.reset();
 }