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

chrome/browser/chromeos/login/users/fake_chrome_user_manager.h

 // Copyright 2015 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_CHROME_USER_MANAGER_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USERS_FAKE_CHROME_USER_MANAGER_H_
 
 #include <map>
 #include <string>
 
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chromeos/login/user_flow.h"
 #include "chrome/browser/chromeos/login/users/user_manager_interface.h"
 #include "components/user_manager/fake_user_manager.h"
 #include "components/user_manager/user.h"
+#include "components/user_manager/user_id.h"
 #include "components/user_manager/user_image/user_image.h"
 #include "components/user_manager/user_manager_base.h"
 
 namespace chromeos {
 
 class FakeSupervisedUserManager;
 
 // Fake chrome user manager with a barebones implementation. Users can be added
 // and set as logged in, and those users can be returned.
 class FakeChromeUserManager : public user_manager::FakeUserManager,
                               public UserManagerInterface {
  public:
   FakeChromeUserManager();
   ~FakeChromeUserManager() override;
 
   // Create and add a kiosk app user.
-  void AddKioskAppUser(const std::string& kiosk_app_username);
+  void AddKioskAppUser(const user_manager::UserID& kiosk_app);
 
   // Create and add a public account user.
-  const user_manager::User* AddPublicAccountUser(const std::string& email);
+  const user_manager::User* AddPublicAccountUser(const user_manager::UserID& user_id);
 
   // Calculates the user name hash and calls UserLoggedIn to login a user.
-  void LoginUser(const std::string& email);
+  void LoginUser(const user_manager::UserID& user_id);
 
   // UserManager overrides.
   user_manager::UserList GetUsersAllowedForMultiProfile() const override;
 
   // user_manager::FakeUserManager override.
-  const user_manager::User* AddUser(const std::string& email) override;
+  const user_manager::User* AddUser(const user_manager::UserID& user_id) override;
 
   // UserManagerInterface implementation.
   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;
-  void SetUserFlow(const std::string& email, UserFlow* flow) override;
+  void SetUserFlow(const user_manager::UserID& user_id, UserFlow* flow) override;
   UserFlow* GetCurrentUserFlow() const override;
-  UserFlow* GetUserFlow(const std::string& email) const override;
-  void ResetUserFlow(const std::string& email) override;
+  UserFlow* GetUserFlow(const user_manager::UserID& user_id) const override;
+  void ResetUserFlow(const user_manager::UserID& user_id) override;
   user_manager::UserList GetUsersAllowedForSupervisedUsersCreation()
       const override;
-  void SwitchActiveUser(const std::string& email) override;
-  const std::string& GetOwnerEmail() const override;
+  void SwitchActiveUser(const user_manager::UserID& user_id) override;
+  const user_manager::UserID& GetOwnerID() const override;
   void SessionStarted() override;
-  void RemoveUser(const std::string& email,
+  void RemoveUser(const user_manager::UserID& user_id,
                   user_manager::RemoveUserDelegate* delegate) override;
   bool FindKnownUserPrefs(const user_manager::UserID& user_id,
                           const base::DictionaryValue** out_value) override;
   void UpdateKnownUserPrefs(const user_manager::UserID& user_id,
                             const base::DictionaryValue& values,
                             bool clear) override;
 
-  void set_owner_email(const std::string& owner_email) {
-    owner_email_ = owner_email;
+  void set_owner_id(const user_manager::UserID& owner_id) {
+    owner_id_ = owner_id;
   }
 
   void set_bootstrap_manager(BootstrapManager* bootstrap_manager) {
     bootstrap_manager_ = bootstrap_manager;
   }
 
   void set_multi_profile_user_controller(
       MultiProfileUserController* controller) {
     multi_profile_user_controller_ = controller;
   }
 
  private:
   // Lazily creates default user flow.
   UserFlow* GetDefaultUserFlow() const;
 
   scoped_ptr<FakeSupervisedUserManager> supervised_user_manager_;
-  std::string owner_email_;
+  user_manager::UserID owner_id_;
 
   BootstrapManager* bootstrap_manager_;
   MultiProfileUserController* multi_profile_user_controller_;
 
-  typedef std::map<std::string, UserFlow*> FlowMap;
+  typedef std::map<user_manager::UserID, UserFlow*> FlowMap;
 
   // Lazy-initialized default flow.
   mutable scoped_ptr<UserFlow> default_flow_;
 
   // Specific flows by user e-mail.
   // Keys should be canonicalized before access.
   FlowMap specific_flows_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeChromeUserManager);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USERS_FAKE_CHROME_USER_MANAGER_H_