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

chrome/browser/supervised_user/chromeos/supervised_user_password_service.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_SUPERVISED_USER_CHROMEOS_SUPERVISED_USER_PASSWORD_SERVICE_H_
 #define CHROME_BROWSER_SUPERVISED_USER_CHROMEOS_SUPERVISED_USER_PASSWORD_SERVICE_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
 #include "chrome/browser/supervised_user/legacy/supervised_user_shared_settings_service.h"
 #include "chrome/browser/supervised_user/supervised_users.h"
 #include "components/keyed_service/core/keyed_service.h"
+#include "components/user_manager/user_id.h"
 
 namespace chromeos {
 
 class SupervisedUserPasswordService : public KeyedService {
  public:
   SupervisedUserPasswordService();
   ~SupervisedUserPasswordService() override;
 
   void Shutdown() override;
 
-  void Init(const std::string& user_id,
+  void Init(const user_manager::UserID& user_id,
             SupervisedUserSharedSettingsService* service);
  private:
   void OnSharedSettingsChange(const std::string& su_id, const std::string& key);
 
   // Cached value from Init().
   // User id of currently logged in supervised user.
-  std::string user_id_;
+  user_manager::UserID user_id_;
   SupervisedUserSharedSettingsService* settings_service_;
 
   scoped_ptr<SupervisedUserSharedSettingsService::ChangeCallbackList::
                  Subscription>
       settings_service_subscription_;
 
   base::WeakPtrFactory<SupervisedUserPasswordService> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserPasswordService);
 };
 
 }  // namespace chromeos
 #endif  // CHROME_BROWSER_SUPERVISED_USER_CHROMEOS_SUPERVISED_USER_PASSWORD_SERVICE_H_