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

chromeos/login/user_names.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 CHROMEOS_LOGIN_USER_NAMES_H_
 #define CHROMEOS_LOGIN_USER_NAMES_H_
 
 #include <string>
 
 #include "chromeos/chromeos_export.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 namespace login {
 
 // Username for stub login when not running on ChromeOS.
-CHROMEOS_EXPORT extern const char* kStubUser;
+CHROMEOS_EXPORT user_manager::UserID GetStubUserID();
 
-// Username for the login screen. It is only used to identify login screen
+// User ID for the login screen. It is only used to identify login screen
 // tries to set default wallpaper. It is not a real user.
-CHROMEOS_EXPORT extern const char* kSignInUser;
+CHROMEOS_EXPORT user_manager::UserID GetSignInUserID();
 
-// Magic e-mail addresses are bad. They exist here because some code already
-// depends on them and it is hard to figure out what. Any user types added in
-// the future should be identified by a new |UserType|, not a new magic e-mail
-// address.
-// Username for Guest session user.
-CHROMEOS_EXPORT extern const char* kGuestUserName;
+// Magic e-mail addresses / user ids are bad. They exist here because some code
+// already depends on them and it is hard to figure out what. Any user types
+// added in the future should be identified by a new |UserType|, not a new magic
+// e-mail address.
+
+// Username for Guest session user (for UserID::Deserialize only).
+CHROMEOS_EXPORT extern const char* kLegacyGuestUserName;
+
+// User id for Guest session user.
+CHROMEOS_EXPORT user_manager::UserID GetGuestUserID();
 
 // Domain that is used for all supervised users.
 CHROMEOS_EXPORT extern const char* kSupervisedUserDomain;
 
 // Canonicalizes a GAIA user ID, accounting for the legacy guest mode user ID
 // which does trips up gaia::CanonicalizeEmail() because it does not contain an
 // @ symbol.
 CHROMEOS_EXPORT std::string CanonicalizeUserID(const std::string& user_id);
 
 }  // namespace login
 
 }  // namespace chromeos
 
 #endif  // CHROMEOS_LOGIN_USER_NAMES_H_