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

chrome/browser/chromeos/login/users/mock_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_MOCK_USER_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USERS_MOCK_USER_MANAGER_H_
 
 #include <string>
 
 #include "base/files/file_path.h"
@@ skipping 92 matching lines @@
   user_manager::UserList GetUnlockUsers() const override;
   const std::string& GetOwnerEmail() const override;
   user_manager::User* GetLoggedInUser() override;
   const user_manager::User* GetActiveUser() const override;
   user_manager::User* GetActiveUser() override;
   const user_manager::User* GetPrimaryUser() const override;
 
   // ChromeUserManager overrides:
   BootstrapManager* GetBootstrapManager() override;
   MultiProfileUserController* GetMultiProfileUserController() override;
-  UserImageManager* GetUserImageManager(const std::string& user_id) override;
+  UserImageManager* GetUserImageManager(const user_manager::UserID& user_id) override;
   SupervisedUserManager* GetSupervisedUserManager() override;
   MOCK_METHOD2(SetUserFlow, void(const std::string&, UserFlow*));
   MOCK_METHOD1(ResetUserFlow, void(const std::string&));
   UserFlow* GetCurrentUserFlow() const override;
   UserFlow* GetUserFlow(const std::string&) const override;
 
   // Sets a new User instance. Users previously created by this MockUserManager
   // become invalid.
-  void SetActiveUser(const std::string& email);
+  void SetActiveUser(const user_manager::UserID& user_id);
 
   // Creates a new public session user. Users previously created by this
   // MockUserManager become invalid.
-  user_manager::User* CreatePublicAccountUser(const std::string& email);
+  user_manager::User* CreatePublicAccountUser(const user_manager::UserID& user_id);
 
   // Creates a new kiosk app user. Users previously created by this
   // MockUserManager become invalid.
-  user_manager::User* CreateKioskAppUser(const std::string& user_id);
+  user_manager::User* CreateKioskAppUser(const user_manager::UserID& user_id);
 
   // Adds a new User instance to the back of the user list. Users previously
   // created by this MockUserManager remain valid.
   void AddUser(const std::string& email);
 
   // Clears the user list and the active user. Users previously created by this
   // MockUserManager become invalid.
   void ClearUserList();
 
   scoped_ptr<UserFlow> user_flow_;
   scoped_ptr<MockUserImageManager> user_image_manager_;
   scoped_ptr<FakeSupervisedUserManager> supervised_user_manager_;
   user_manager::UserList user_list_;
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USERS_MOCK_USER_MANAGER_H_