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

chrome/browser/chromeos/login/users/user_manager_interface.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_USER_MANAGER_INTERFACE_H_
 #define CHROME_BROWSER_CHROMEOS_LOGIN_USERS_USER_MANAGER_INTERFACE_H_
 
 #include "base/basictypes.h"
 #include "components/user_manager/user.h"
 #include "components/user_manager/user_type.h"
 
 namespace chromeos {
 
 class BootstrapManager;
 class MultiProfileUserController;
 class SupervisedUserManager;
 class UserFlow;
 class UserImageManager;
 
 // Chrome specific add-ons interface for the UserManager.
 class UserManagerInterface {
  public:
   UserManagerInterface() {}
   virtual ~UserManagerInterface() {}
 
   virtual BootstrapManager* GetBootstrapManager() = 0;
   virtual MultiProfileUserController* GetMultiProfileUserController() = 0;
-  virtual UserImageManager* GetUserImageManager(const std::string& user_id) = 0;
+  virtual UserImageManager* GetUserImageManager(const user_manager::UserID& user_id) = 0;
   virtual SupervisedUserManager* GetSupervisedUserManager() = 0;
 
   // Method that allows to set |flow| for user identified by |user_id|.
   // Flow should be set before login attempt.
   // Takes ownership of the |flow|, |flow| will be deleted in case of login
   // failure.
-  virtual void SetUserFlow(const std::string& user_id, UserFlow* flow) = 0;
+  virtual void SetUserFlow(const user_manager::UserID& user_id, UserFlow* flow) = 0;
 
   // Return user flow for current user. Returns instance of DefaultUserFlow if
   // no flow was defined for current user, or user is not logged in.
   // Returned value should not be cached.
   virtual UserFlow* GetCurrentUserFlow() const = 0;
 
   // Return user flow for user identified by |user_id|. Returns instance of
   // DefaultUserFlow if no flow was defined for user.
   // Returned value should not be cached.
-  virtual UserFlow* GetUserFlow(const std::string& user_id) const = 0;
+  virtual UserFlow* GetUserFlow(const user_manager::UserID& user_id) const = 0;
 
   // Resets user flow for user identified by |user_id|.
-  virtual void ResetUserFlow(const std::string& user_id) = 0;
+  virtual void ResetUserFlow(const user_manager::UserID& user_id) = 0;
 
   // Returns list of users allowed for supervised user creation.
   // Returns an empty list in cases when supervised user creation or adding new
   // users is restricted.
   virtual user_manager::UserList GetUsersAllowedForSupervisedUsersCreation()
       const = 0;
 
   DISALLOW_COPY_AND_ASSIGN(UserManagerInterface);
 };
 
 }  // namespace chromeos
 
 #endif  // CHROME_BROWSER_CHROMEOS_LOGIN_USERS_USER_MANAGER_INTERFACE_H_