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

chrome/browser/chromeos/login/users/fake_supervised_user_manager.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_USERS_FAKE_SUPERVISED_USER_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USERS_FAKE_SUPERVISED_USER_MANAGER_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "chrome/browser/chromeos/login/users/supervised_user_manager.h"
 
+namespace user_manager {
+class UserID;
+} // namespace user_manager
+
 namespace chromeos {
 
 // Fake supervised user manager with a barebones implementation.
 class FakeSupervisedUserManager : public SupervisedUserManager {
  public:
   FakeSupervisedUserManager();
   ~FakeSupervisedUserManager() override;
 
-  bool HasSupervisedUsers(const std::string& manager_id) const override;
+  bool HasSupervisedUsers(const user_manager::UserID& manager_id) const override;
   const user_manager::User* CreateUserRecord(
-      const std::string& manager_id,
-      const std::string& local_user_id,
+      const user_manager::UserID& manager_id,
+      const user_manager::UserID& local_user_id,
       const std::string& sync_user_id,
       const base::string16& display_name) override;
   std::string GenerateUserId() override;
   const user_manager::User* FindByDisplayName(
       const base::string16& display_name) const override;
   const user_manager::User* FindBySyncId(
       const std::string& sync_id) const override;
-  std::string GetUserSyncId(const std::string& user_id) const override;
+  std::string GetUserSyncId(const user_manager::UserID& user_id) const override;
   base::string16 GetManagerDisplayName(
-      const std::string& user_id) const override;
-  std::string GetManagerUserId(const std::string& user_id) const override;
-  std::string GetManagerDisplayEmail(const std::string& user_id) const override;
+      const user_manager::UserID& user_id) const override;
+  const user_manager::UserID& GetManagerUserId(const user_manager::UserID& user_id) const override;
+  std::string GetManagerDisplayEmail(const user_manager::UserID& user_id) const override;
   void StartCreationTransaction(const base::string16& display_name) override {}
-  void SetCreationTransactionUserId(const std::string& user_id) override {}
+  void SetCreationTransactionUserId(const user_manager::UserID& user_id) override {}
   void CommitCreationTransaction() override {}
   SupervisedUserAuthentication* GetAuthentication() override;
-  void GetPasswordInformation(const std::string& user_id,
+  void GetPasswordInformation(const user_manager::UserID& user_id,
                               base::DictionaryValue* result) override {}
   void SetPasswordInformation(
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       const base::DictionaryValue* password_info) override {}
   void LoadSupervisedUserToken(Profile* profile,
                                const LoadTokenCallback& callback) override;
   void ConfigureSyncWithToken(Profile* profile,
                               const std::string& token) override {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FakeSupervisedUserManager);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USERS_FAKE_SUPERVISED_USER_MANAGER_H_