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

ash/system/tray/system_tray_item.h

Index: ash/system/tray/system_tray_item.h
diff --git a/ash/system/tray/system_tray_item.h b/ash/system/tray/system_tray_item.h
index 88f586d82bd07eb337ef59f738a10a66852702ff..5e8e4aaebfdd26423b784bd6cae894a0757210ae 100644
--- a/ash/system/tray/system_tray_item.h
+++ b/ash/system/tray/system_tray_item.h
@@ -44,6 +44,11 @@ class ASH_EXPORT SystemTrayItem {
   virtual void DestroyDefaultView() = 0;
   virtual void DestroyDetailedView() = 0;
 
+  // Updates the tray view (if applicable) when the user's login status changes.
+  // It is not necessary the update the default or detailed view, since the
+  // default/detailed popup is closed when login status changes.
+  virtual void UpdateAfterLoginStatusChange(user::LoginStatus status) = 0;
+
   // Pops up the detailed view for this item. An item can request to show its
   // detailed view using this function (e.g. from an observer callback when
   // something, e.g. volume, network availability etc. changes). If