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

chromeos/login/user_names.h

Index: chromeos/login/user_names.h
diff --git a/chromeos/login/user_names.h b/chromeos/login/user_names.h
index 718d7aeb797e549c8c662579cdb4bc94c3515b40..fb980a822241a91af3403c590eb7c6b1e0ddb782 100644
--- a/chromeos/login/user_names.h
+++ b/chromeos/login/user_names.h
@@ -9,23 +9,31 @@
 
 #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;
-
-// 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;
+CHROMEOS_EXPORT user_manager::UserID GetSignInUserID();
+
+// 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;