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

chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.h

Index: chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.h
diff --git a/chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.h b/chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.h
index f3597f671587d60ba536a3f287f03154993e9c13..d10083448e83bc12866c9a6cf4edf8a87d1da69c 100644
--- a/chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.h
+++ b/chrome/browser/supervised_user/legacy/supervised_user_registration_utility_stub.h
@@ -11,6 +11,7 @@
 #include "base/compiler_specific.h"
 #include "base/strings/string16.h"
 #include "chrome/browser/supervised_user/legacy/supervised_user_registration_utility.h"
+#include "components/user_manager/user_id.h"
 #include "google_apis/gaia/google_service_auth_error.h"
 
 class SupervisedUserRegistrationUtilityStub
@@ -19,13 +20,13 @@ class SupervisedUserRegistrationUtilityStub
   SupervisedUserRegistrationUtilityStub();
   ~SupervisedUserRegistrationUtilityStub() override;
 
-  void Register(const std::string& supervised_user_id,
+  void Register(const user_manager::UserID& supervised_user_id,
                 const SupervisedUserRegistrationInfo& info,
                 const RegistrationCallback& callback) override;
 
   bool register_was_called() { return register_was_called_; }
 
-  std::string supervised_user_id() { return supervised_user_id_; }
+  user_manager::UserID supervised_user_id() { return supervised_user_id_; }
 
   base::string16 display_name() { return display_name_; }
 
@@ -37,7 +38,7 @@ class SupervisedUserRegistrationUtilityStub
  private:
   RegistrationCallback callback_;
   bool register_was_called_;
-  std::string supervised_user_id_;
+  user_manager::UserID supervised_user_id_;
   base::string16 display_name_;
   std::string master_key_;