ash: Refresh the tray items, instead of recreating them, when login status changes.

ash/system/brightness/tray_brightness.cc

Index: ash/system/brightness/tray_brightness.cc
diff --git a/ash/system/brightness/tray_brightness.cc b/ash/system/brightness/tray_brightness.cc
index c59da84f31197a1f7afebdb06e11c6b1e21130fd..fd75528d6500e5548ed9e0ecfb401fcbf65680c5 100644
--- a/ash/system/brightness/tray_brightness.cc
+++ b/ash/system/brightness/tray_brightness.cc
@@ -184,6 +184,9 @@ void TrayBrightness::DestroyDetailedView() {
     brightness_view_.reset();
 }
 
+void TrayBrightness::UpdateAfterLoginStatusChange(user::LoginStatus status) {
+}
+
 void TrayBrightness::OnBrightnessChanged(double percent, bool user_initiated) {
   current_percent_ = percent;
   got_current_percent_ = true;