Make UserActivityDetector a singleton

chrome/browser/chromeos/login/ui/webui_login_display.cc

Index: chrome/browser/chromeos/login/ui/webui_login_display.cc
diff --git a/chrome/browser/chromeos/login/ui/webui_login_display.cc b/chrome/browser/chromeos/login/ui/webui_login_display.cc
index 3eee06f6f597dc1c019cec2dea49ca6788a8052e..a2608eecd858d817a0b068afa3db6a76339e9027 100644
--- a/chrome/browser/chromeos/login/ui/webui_login_display.cc
+++ b/chrome/browser/chromeos/login/ui/webui_login_display.cc
@@ -4,7 +4,6 @@
 
 #include "chrome/browser/chromeos/login/ui/webui_login_display.h"
 
-#include "ash/shell.h"
 #include "chrome/browser/chromeos/accessibility/accessibility_manager.h"
 #include "chrome/browser/chromeos/login/lock/screen_locker.h"
 #include "chrome/browser/chromeos/login/screens/chrome_user_selection_screen.h"
@@ -35,12 +34,9 @@ namespace chromeos {
 WebUILoginDisplay::~WebUILoginDisplay() {
   if (webui_handler_)
     webui_handler_->ResetSigninScreenHandlerDelegate();
-#if !defined(USE_ATHENA)
-  wm::UserActivityDetector* activity_detector = ash::Shell::GetInstance()->
-    user_activity_detector();
+  wm::UserActivityDetector* activity_detector = wm::UserActivityDetector::Get();
   if (activity_detector->HasObserver(this))
     activity_detector->RemoveObserver(this);
-#endif
 }
 
 // LoginDisplay implementation: ------------------------------------------------
@@ -71,12 +67,9 @@ void WebUILoginDisplay::Init(const user_manager::UserList& users,
   show_users_ = show_users;
   show_new_user_ = show_new_user;
 
-#if !defined(USE_ATHENA)
-  wm::UserActivityDetector* activity_detector = ash::Shell::GetInstance()->
-      user_activity_detector();
+  wm::UserActivityDetector* activity_detector = wm::UserActivityDetector::Get();
   if (!activity_detector->HasObserver(this))
     activity_detector->AddObserver(this);
-#endif
 }
 
 // ---- Common methods