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

chrome/browser/supervised_user/supervised_user_service.cc

Index: chrome/browser/supervised_user/supervised_user_service.cc
diff --git a/chrome/browser/supervised_user/supervised_user_service.cc b/chrome/browser/supervised_user/supervised_user_service.cc
index 182d415ba9375f9d7110c9da005383470567e2f9..6daf76bb6bb807b9e30c963a90c7adc243099f79 100644
--- a/chrome/browser/supervised_user/supervised_user_service.cc
+++ b/chrome/browser/supervised_user/supervised_user_service.cc
@@ -333,7 +333,7 @@ std::string SupervisedUserService::GetCustodianEmailAddress() const {
     custodian_email = chromeos::ChromeUserManager::Get()
         ->GetSupervisedUserManager()
         ->GetManagerDisplayEmail(
-            user_manager::UserManager::Get()->GetActiveUser()->email());
+            user_manager::UserManager::Get()->GetActiveUser()->GetUserID());
   }
 #endif
   return custodian_email;
@@ -347,7 +347,7 @@ std::string SupervisedUserService::GetCustodianName() const {
     name = base::UTF16ToUTF8(chromeos::ChromeUserManager::Get()
         ->GetSupervisedUserManager()
         ->GetManagerDisplayName(
-            user_manager::UserManager::Get()->GetActiveUser()->email()));
+            user_manager::UserManager::Get()->GetActiveUser()->GetUserID()));
   }
 #endif
   return name.empty() ? GetCustodianEmailAddress() : name;
@@ -849,7 +849,7 @@ void SupervisedUserService::SetActive(bool active) {
 void SupervisedUserService::RegisterAndInitSync(
     SupervisedUserRegistrationUtility* registration_utility,
     Profile* custodian_profile,
-    const std::string& supervised_user_id,
+    const user_manager::UserID& supervised_user_id,
     const AuthErrorCallback& callback) {
   DCHECK(ProfileIsSupervised());
   DCHECK(!custodian_profile->IsSupervised());