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

chrome/browser/supervised_user/legacy/supervised_user_sync_service.h

Index: chrome/browser/supervised_user/legacy/supervised_user_sync_service.h
diff --git a/chrome/browser/supervised_user/legacy/supervised_user_sync_service.h b/chrome/browser/supervised_user/legacy/supervised_user_sync_service.h
index abde2ac59f2935a7557aea2d6d6f3468de553c30..3653fd76b1be8bbc20bb0421a69e5a423c19bc58 100644
--- a/chrome/browser/supervised_user/legacy/supervised_user_sync_service.h
+++ b/chrome/browser/supervised_user/legacy/supervised_user_sync_service.h
@@ -25,6 +25,10 @@ namespace user_prefs {
 class PrefRegistrySyncable;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 class PrefService;
 class Profile;
 
@@ -70,20 +74,20 @@ class SupervisedUserSyncService : public KeyedService,
   void AddObserver(SupervisedUserSyncServiceObserver* observer);
   void RemoveObserver(SupervisedUserSyncServiceObserver* observer);
 
-  void AddSupervisedUser(const std::string& id,
+  void AddSupervisedUser(const user_manager::UserID& id,
                          const std::string& name,
                          const std::string& master_key,
                          const std::string& signature_key,
                          const std::string& encryption_key,
                          int avatar_index);
-  void UpdateSupervisedUser(const std::string& id,
+  void UpdateSupervisedUser(const user_manager::UserID& id,
                             const std::string& name,
                             const std::string& master_key,
                             const std::string& signature_key,
                             const std::string& encryption_key,
                             int avatar_index);
 
-  void DeleteSupervisedUser(const std::string& id);
+  void DeleteSupervisedUser(const user_manager::UserID& id);
 
   // Updates the supervised user avatar only if the supervised user has
   // no avatar and |avatar_index| is set to some value other than
@@ -93,9 +97,9 @@ class SupervisedUserSyncService : public KeyedService,
   // |ClearSupervisedUserAvatar()|.
   // Returns true if the avatar value is changed (either updated or cleared)
   // and false otherwise.
-  bool UpdateSupervisedUserAvatarIfNeeded(const std::string& id,
+  bool UpdateSupervisedUserAvatarIfNeeded(const user_manager::UserID& id,
                                           int avatar_index);
-  void ClearSupervisedUserAvatar(const std::string& id);
+  void ClearSupervisedUserAvatar(const user_manager::UserID& id);
 
   // Returns a dictionary containing all supervised users supervised by this
   // custodian. This method should only be called once this service has started
@@ -141,7 +145,7 @@ class SupervisedUserSyncService : public KeyedService,
       const std::string& encryption_key,
       int avatar_index);
 
-  void UpdateSupervisedUserImpl(const std::string& id,
+  void UpdateSupervisedUserImpl(const user_manager::UserID& id,
                                 const std::string& name,
                                 const std::string& master_key,
                                 const std::string& signature_key,
@@ -149,7 +153,7 @@ class SupervisedUserSyncService : public KeyedService,
                                 int avatar_index,
                                 bool add_user);
 
-  void NotifySupervisedUserAcknowledged(const std::string& supervised_user_id);
+  void NotifySupervisedUserAcknowledged(const user_manager::UserID& supervised_user_id);
   void NotifySupervisedUsersSyncingStopped();
   void NotifySupervisedUsersChanged();