We should use UserID object to identify users instead of username.

chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.cc

Index: chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.cc
diff --git a/chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.cc b/chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.cc
index b515a90dd7041c4705bc362abe2cdc18730e6391..137f94725007945322d3d8b2d6e7c3fb1a12b62f 100644
--- a/chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.cc
+++ b/chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.cc
@@ -11,7 +11,7 @@
 
 MultiUserNotificationBlockerChromeOS::MultiUserNotificationBlockerChromeOS(
     message_center::MessageCenter* message_center,
-    const std::string& initial_user_id)
+    const user_manager::UserID& initial_user_id)
     : NotificationBlocker(message_center),
       active_user_id_(initial_user_id) {
 }
@@ -27,7 +27,7 @@ bool MultiUserNotificationBlockerChromeOS::ShouldShowNotification(
   if (ash::system_notifier::IsAshSystemNotifier(notifier_id))
     return true;
 
-  return notifier_id.profile_id == active_user_id_;
+  return notifier_id.profile_id == active_user_id_.GetUserEmail();
 }
 
 bool MultiUserNotificationBlockerChromeOS::ShouldShowNotificationAsPopup(
@@ -36,13 +36,13 @@ bool MultiUserNotificationBlockerChromeOS::ShouldShowNotificationAsPopup(
 }
 
 void MultiUserNotificationBlockerChromeOS::ActiveUserChanged(
-    const std::string& user_id) {
+    const user_manager::UserID& user_id) {
   if (active_user_id_ == user_id)
     return;
 
   quiet_modes_[active_user_id_] = message_center()->IsQuietMode();
   active_user_id_ = user_id;
-  std::map<std::string, bool>::const_iterator iter =
+  std::map<user_manager::UserID, bool>::const_iterator iter =
       quiet_modes_.find(active_user_id_);
   if (iter != quiet_modes_.end() &&
       iter->second != message_center()->IsQuietMode()) {