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

chrome/browser/supervised_user/chromeos/manager_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_MANAGER_PASSWORD_SERVICE_H_
 #define CHROME_BROWSER_SUPERVISED_USER_CHROMEOS_MANAGER_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/legacy/supervised_user_sync_service.h"
 #include "chrome/browser/supervised_user/supervised_users.h"
 #include "chromeos/login/auth/extended_authenticator.h"
 #include "components/keyed_service/core/keyed_service.h"
+#include "components/user_manager/user_id.h"
+
 
 namespace chromeos {
 
 class UserContext;
 
 // Handles supervised user password change that is detected while manager is
 // signed in.
 // It uses manager's master key to authorize update of supervised user's key.
 // Edge case: Pre-M35 supervised users don't have correct labels for keys.
 // After new supervised user key is added, migration is done in following way:
 // 1) Master key is added with correct label
 // 2) Old supervised user's key is deleted.
 // 3) Old master key is deleted.
 class ManagerPasswordService
     : public KeyedService,
       public chromeos::ExtendedAuthenticator::NewAuthStatusConsumer {
  public:
   ManagerPasswordService();
   ~ManagerPasswordService() override;
 
   void Shutdown() override;
 
-  void Init(const std::string& user_id,
+  void Init(const user_manager::UserID& user_id,
             SupervisedUserSyncService* user_service,
             SupervisedUserSharedSettingsService* service);
 
   // chromeos::ExtendedAuthenticator::AuthStatusConsumer overrides:
   void OnAuthenticationFailure(ExtendedAuthenticator::AuthState state) override;
 
  private:
   void OnSharedSettingsChange(const std::string& su_id, const std::string& key);
   void GetSupervisedUsersCallback(
       const std::string& sync_su_id,
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       scoped_ptr<base::DictionaryValue> password_data,
       const base::DictionaryValue* supervised_users);
   void OnAddKeySuccess(const UserContext& master_key_context,
-                       const std::string& user_id,
+                       const user_manager::UserID& user_id,
                        scoped_ptr<base::DictionaryValue> password_data);
   void OnKeyTransformedIfNeeded(const UserContext& master_key_context);
   void OnNewManagerKeySuccess(const UserContext& master_key_context);
   void OnOldSupervisedUserKeyDeleted(const UserContext& master_key_context);
   void OnOldManagerKeyDeleted(const UserContext& master_key_context);
 
   // Cached value from Init().
   // User id of currently logged in user, that have supervised users on device.
-  std::string user_id_;
+  user_manager::UserID user_id_;
   SupervisedUserSyncService* user_service_;
   SupervisedUserSharedSettingsService* settings_service_;
 
   scoped_ptr<SupervisedUserSharedSettingsService::ChangeCallbackList::
                  Subscription>
       settings_service_subscription_;
 
   scoped_refptr<ExtendedAuthenticator> authenticator_;
 
   base::WeakPtrFactory<ManagerPasswordService> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(ManagerPasswordService);
 };
 
 }  // namespace chromeos
 #endif  // CHROME_BROWSER_SUPERVISED_USER_CHROMEOS_MANAGER_PASSWORD_SERVICE_H_