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

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

 // 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.
 
 #ifndef CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_NOTIFICATION_BLOCKER_CHROMEOS_H_
 #define CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_NOTIFICATION_BLOCKER_CHROMEOS_H_
 
 #include <map>
 #include <set>
 #include <string>
 
+#include "components/user_manager/user_id.h"
 #include "ui/message_center/notification_blocker.h"
 
 // A notification blocker for per-profile stream switching. Owned and controlled
 // by MultiUserWindowManagerChromeOS.
 class MultiUserNotificationBlockerChromeOS
     : public message_center::NotificationBlocker {
  public:
   MultiUserNotificationBlockerChromeOS(
       message_center::MessageCenter* message_center,
-      const std::string& initial_user_id);
+      const user_manager::UserID& initial_user_id);
   ~MultiUserNotificationBlockerChromeOS() override;
 
   // Called by MultiUserWindowManager when the active user has changed.
-  void ActiveUserChanged(const std::string& user_id);
+  void ActiveUserChanged(const user_manager::UserID& user_id);
 
   // message_center::NotificationBlocker overrides:
   bool ShouldShowNotification(
       const message_center::NotifierId& notifier_id) const override;
   bool ShouldShowNotificationAsPopup(
       const message_center::NotifierId& notifier_id) const override;
 
  private:
   // Returns true if this blocker is actively working.
   bool IsActive() const;
 
-  std::string active_user_id_;
-  std::map<std::string, bool> quiet_modes_;
+  user_manager::UserID active_user_id_;
+  std::map<user_manager::UserID, bool> quiet_modes_;
 
   DISALLOW_COPY_AND_ASSIGN(MultiUserNotificationBlockerChromeOS);
 };
 
 #endif  // CHROME_BROWSER_UI_ASH_MULTI_USER_MULTI_USER_NOTIFICATION_BLOCKER_CHROMEOS_H_