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

chrome/browser/supervised_user/supervised_user_service.h

Index: chrome/browser/supervised_user/supervised_user_service.h
diff --git a/chrome/browser/supervised_user/supervised_user_service.h b/chrome/browser/supervised_user/supervised_user_service.h
index 1f10b59b7b43b18cfaec5861fafc12e45badc39a..1f39af896923293453b39292c06bbd27b2e4d01d 100644
--- a/chrome/browser/supervised_user/supervised_user_service.h
+++ b/chrome/browser/supervised_user/supervised_user_service.h
@@ -58,6 +58,10 @@ namespace user_prefs {
 class PrefRegistrySyncable;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 // This class handles all the information related to a given supervised profile
 // (e.g. the installed content packs, the default URL filtering behavior, or
 // manual whitelist/blacklist overrides).
@@ -144,7 +148,7 @@ class SupervisedUserService : public KeyedService,
   void RegisterAndInitSync(
       SupervisedUserRegistrationUtility* registration_utility,
       Profile* custodian_profile,
-      const std::string& supervised_user_id,
+      const user_manager::UserID& supervised_user_id,
       const AuthErrorCallback& callback);
 
   void AddNavigationBlockedCallback(const NavigationBlockedCallback& callback);