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

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

Index: chrome/browser/chromeos/login/supervised/supervised_user_authentication.h
diff --git a/chrome/browser/chromeos/login/supervised/supervised_user_authentication.h b/chrome/browser/chromeos/login/supervised/supervised_user_authentication.h
index c636abf70fde3f90c31f4b11d493b630db720b61..d8e44700707b804f8931d5f1d9a4ceba567aeb54 100644
--- a/chrome/browser/chromeos/login/supervised/supervised_user_authentication.h
+++ b/chrome/browser/chromeos/login/supervised/supervised_user_authentication.h
@@ -61,31 +61,31 @@ class SupervisedUserAuthentication {
 
   // Fills |password_data| with |password|-specific data for |user_id|,
   // depending on target schema. Does not affect Local State.
-  bool FillDataForNewUser(const std::string& user_id,
+  bool FillDataForNewUser(const user_manager::UserID& user_id,
                           const std::string& password,
                           base::DictionaryValue* password_data,
                           base::DictionaryValue* extra_data);
 
   // Stores |password_data| for |user_id| in Local State. Only public parts
   // of |password_data| will be stored.
-  void StorePasswordData(const std::string& user_id,
+  void StorePasswordData(const user_manager::UserID& user_id,
                          const base::DictionaryValue& password_data);
 
-  bool NeedPasswordChange(const std::string& user_id,
+  bool NeedPasswordChange(const user_manager::UserID& user_id,
                           const base::DictionaryValue* password_data);
 
   // Checks if given user should update password upon signin.
-  bool HasScheduledPasswordUpdate(const std::string& user_id);
-  void ClearScheduledPasswordUpdate(const std::string& user_id);
+  bool HasScheduledPasswordUpdate(const user_manager::UserID& user_id);
+  void ClearScheduledPasswordUpdate(const user_manager::UserID& user_id);
 
   // Checks if password was migrated to new schema by supervised user.
   // In this case it does not have encryption key, and should be updated by
   // manager even if password versions match.
-  bool HasIncompleteKey(const std::string& user_id);
-  void MarkKeyIncomplete(const std::string& user_id, bool incomplete);
+  bool HasIncompleteKey(const user_manager::UserID& user_id);
+  void MarkKeyIncomplete(const user_manager::UserID& user_id, bool incomplete);
 
   // Loads password data stored by ScheduleSupervisedPasswordChange.
-  void LoadPasswordUpdateData(const std::string& user_id,
+  void LoadPasswordUpdateData(const user_manager::UserID& user_id,
                               const PasswordDataCallback& success_callback,
                               const base::Closure& failure_callback);
 
@@ -95,11 +95,11 @@ class SupervisedUserAuthentication {
 
   // Called by supervised user to store password data for migration upon signin.
   void ScheduleSupervisedPasswordChange(
-      const std::string& supervised_user_id,
+      const user_manager::UserID& supervised_user_id,
       const base::DictionaryValue* password_data);
 
   // Utility method that gets schema version for |user_id| from Local State.
-  Schema GetPasswordSchema(const std::string& user_id);
+  Schema GetPasswordSchema(const user_manager::UserID& user_id);
 
   static std::string BuildPasswordSignature(
       const std::string& password,