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

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

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/ui/ash/multi_user/multi_user_notification_blocker_chromeos.h"
 
 #include "ash/system/system_notifier.h"
 #include "chrome/browser/ui/ash/multi_user/multi_user_window_manager.h"
 #include "ui/message_center/message_center.h"
 #include "ui/message_center/notifier_settings.h"
 
 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) {
 }
 
 MultiUserNotificationBlockerChromeOS::~MultiUserNotificationBlockerChromeOS() {
 }
 
 bool MultiUserNotificationBlockerChromeOS::ShouldShowNotification(
     const message_center::NotifierId& notifier_id) const {
   if (!IsActive())
     return true;
 
   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(
     const message_center::NotifierId& notifier_id) const {
   return ShouldShowNotification(notifier_id);
 }
 
 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()) {
     message_center()->SetQuietMode(iter->second);
   }
   NotifyBlockingStateChanged();
 }
 
 bool MultiUserNotificationBlockerChromeOS::IsActive() const {
   return chrome::MultiUserWindowManager::GetMultiProfileMode() ==
       chrome::MultiUserWindowManager::MULTI_PROFILE_MODE_SEPARATED;
 }