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

chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.h

Index: chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.h
diff --git a/chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.h b/chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.h
index d51a8edb7246685ba84a066d758aad8cf0f72063..6d059a9b618109ebdd6727d206ad91d00ceda958 100644
--- a/chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.h
+++ b/chrome/browser/chromeos/login/supervised/supervised_user_creation_screen.h
@@ -20,6 +20,10 @@
 
 class Profile;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace chromeos {
 
 class ErrorScreensHistogramHelper;
@@ -70,7 +74,7 @@ class SupervisedUserCreationScreen
 
   // SupervisedUserSyncServiceObserver implementation
   void OnSupervisedUserAcknowledged(
-      const std::string& supervised_user_id) override {}
+      const user_manager::UserID& supervised_user_id) override {}
   void OnSupervisedUsersSyncingStopped() override {}
   void OnSupervisedUsersChanged() override;
 
@@ -85,10 +89,10 @@ class SupervisedUserCreationScreen
   void CreateSupervisedUser(
       const base::string16& display_name,
       const std::string& supervised_user_password) override;
-  void ImportSupervisedUser(const std::string& user_id) override;
-  void ImportSupervisedUserWithPassword(const std::string& user_id,
+  void ImportSupervisedUser(const user_manager::UserID& user_id) override;
+  void ImportSupervisedUserWithPassword(const user_manager::UserID& user_id,
                                         const std::string& password) override;
-  void AuthenticateManager(const std::string& manager_id,
+  void AuthenticateManager(const user_manager::UserID& manager_id,
                            const std::string& manager_password) override;
   void AbortFlow() override;
   void FinishFlow() override;