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

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

Index: chrome/browser/chromeos/login/users/mock_user_manager.cc
diff --git a/chrome/browser/chromeos/login/users/mock_user_manager.cc b/chrome/browser/chromeos/login/users/mock_user_manager.cc
index a98aa43eb68b8140e91d7c5171e867cda2a6b7a3..2f34eb611276ec9f9b28d64acd40fd0157b71c79 100644
--- a/chrome/browser/chromeos/login/users/mock_user_manager.cc
+++ b/chrome/browser/chromeos/login/users/mock_user_manager.cc
@@ -113,15 +113,15 @@ user_manager::User* MockUserManager::CreatePublicAccountUser(
 }
 
 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);
 }