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

chrome/browser/chromeos/login/supervised/supervised_user_authentication.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_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_AUTHENTICATION_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_AUTHENTICATION_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "base/memory/weak_ptr.h"
 #include "base/strings/string16.h"
@@ skipping 43 matching lines @@
 
   // Returns current schema for whole ChromeOS. It defines if users with older
   // schema should be migrated somehow.
   Schema GetStableSchema();
 
   // Transforms key according to schema specified in Local State.
   UserContext TransformKey(const UserContext& context);
 
   // Fills |password_data| with |password|-specific data for |user_id|,
   // depending on target schema. Does not affect Local State.
-  bool FillDataForNewUser(const std::string& user_id,
+  bool FillDataForNewUser(const user_manager::UserID& user_id,
                           const std::string& password,
                           base::DictionaryValue* password_data,
                           base::DictionaryValue* extra_data);
 
   // Stores |password_data| for |user_id| in Local State. Only public parts
   // of |password_data| will be stored.
-  void StorePasswordData(const std::string& user_id,
+  void StorePasswordData(const user_manager::UserID& user_id,
                          const base::DictionaryValue& password_data);
 
-  bool NeedPasswordChange(const std::string& user_id,
+  bool NeedPasswordChange(const user_manager::UserID& user_id,
                           const base::DictionaryValue* password_data);
 
   // Checks if given user should update password upon signin.
-  bool HasScheduledPasswordUpdate(const std::string& user_id);
-  void ClearScheduledPasswordUpdate(const std::string& user_id);
+  bool HasScheduledPasswordUpdate(const user_manager::UserID& user_id);
+  void ClearScheduledPasswordUpdate(const user_manager::UserID& user_id);
 
   // Checks if password was migrated to new schema by supervised user.
   // In this case it does not have encryption key, and should be updated by
   // manager even if password versions match.
-  bool HasIncompleteKey(const std::string& user_id);
-  void MarkKeyIncomplete(const std::string& user_id, bool incomplete);
+  bool HasIncompleteKey(const user_manager::UserID& user_id);
+  void MarkKeyIncomplete(const user_manager::UserID& user_id, bool incomplete);
 
   // Loads password data stored by ScheduleSupervisedPasswordChange.
-  void LoadPasswordUpdateData(const std::string& user_id,
+  void LoadPasswordUpdateData(const user_manager::UserID& user_id,
                               const PasswordDataCallback& success_callback,
                               const base::Closure& failure_callback);
 
   // Creates a random string that can be used as a master key for managed
   // user's homedir.
   std::string GenerateMasterKey();
 
   // Called by supervised user to store password data for migration upon signin.
   void ScheduleSupervisedPasswordChange(
-      const std::string& supervised_user_id,
+      const user_manager::UserID& supervised_user_id,
       const base::DictionaryValue* password_data);
 
   // Utility method that gets schema version for |user_id| from Local State.
-  Schema GetPasswordSchema(const std::string& user_id);
+  Schema GetPasswordSchema(const user_manager::UserID& user_id);
 
   static std::string BuildPasswordSignature(
       const std::string& password,
       int revision,
       const std::string& base64_signature_key);
 
  private:
   SupervisedUserManager* owner_;
 
   // Target schema version. Affects migration process and new user creation.
   Schema stable_schema_;
 
 
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserAuthentication);
 };
 
 } // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_SUPERVISED_SUPERVISED_USER_AUTHENTICATION_H_