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

chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h

Index: chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h
diff --git a/chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h b/chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h
index 561a8bde1bdee2aed3662e2d586898f5d5e523ff..75709982daf293f19603aa364569840d06cfb0d5 100644
--- a/chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h
+++ b/chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h
@@ -31,6 +31,10 @@ namespace browser_sync {
 class DeviceInfo;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 // Structure to store registration information.
 struct SupervisedUserRegistrationInfo {
   SupervisedUserRegistrationInfo(const base::string16& name, int avatar_index);
@@ -62,7 +66,7 @@ class SupervisedUserRegistrationUtility {
   // Creates SupervisedUserRegistrationUtility for a given |profile|.
   static scoped_ptr<SupervisedUserRegistrationUtility> Create(Profile* profile);
 
-  static std::string GenerateNewSupervisedUserId();
+  static user_manager::UserID GenerateNewSupervisedUserId();
 
   // Registers a new supervised user with the server. |supervised_user_id| is a
   // new unique ID for the new supervised user. If its value is the same as that
@@ -72,7 +76,7 @@ class SupervisedUserRegistrationUtility {
   // the user and his avatar. |callback| is called with the result of the
   // registration. We use the info here and not the profile, because on Chrome
   // OS the profile of the supervised user does not yet exist.
-  virtual void Register(const std::string& supervised_user_id,
+  virtual void Register(const user_manager::UserID& supervised_user_id,
                         const SupervisedUserRegistrationInfo& info,
                         const RegistrationCallback& callback) = 0;