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

chrome/browser/chromeos/login/users/mock_user_manager.cc

 // 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.
 
 #include "chrome/browser/chromeos/login/users/mock_user_manager.h"
 
 #include "base/task_runner.h"
 #include "chrome/browser/chromeos/login/users/fake_supervised_user_manager.h"
 #include "chrome/browser/chromeos/profiles/profile_helper.h"
 
@@ skipping 95 matching lines @@
 user_manager::User* MockUserManager::CreatePublicAccountUser(
     const std::string& email) {
   ClearUserList();
   user_manager::User* user = user_manager::User::CreatePublicAccountUser(email);
   user_list_.push_back(user);
   ProfileHelper::Get()->SetProfileToUserMappingForTesting(user);
   return user_list_.back();
 }
 
 user_manager::User* MockUserManager::CreateKioskAppUser(
-    const std::string& email) {
+    const UserID& user_id) {
   ClearUserList();
-  user_list_.push_back(user_manager::User::CreateKioskAppUser(email));
+  user_list_.push_back(user_manager::User::CreateKioskAppUser(user_id));
   ProfileHelper::Get()->SetProfileToUserMappingForTesting(user_list_.back());
   return user_list_.back();
 }
 
-void MockUserManager::AddUser(const std::string& email) {
-  user_manager::User* user = user_manager::User::CreateRegularUser(email);
+void MockUserManager::AddUser(const user_manager::UserID& user_id) {
+  user_manager::User* user = user_manager::User::CreateRegularUser(user_id);
   user_list_.push_back(user);
   ProfileHelper::Get()->SetProfileToUserMappingForTesting(user);
 }
 
 void MockUserManager::ClearUserList() {
   // Can't use STLDeleteElements because of the protected destructor of User.
   user_manager::UserList::iterator user;
   for (user = user_list_.begin(); user != user_list_.end(); ++user)
     delete *user;
   user_list_.clear();
 }
 
 }  // namespace chromeos