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

chrome/browser/supervised_user/chromeos/manager_password_service.h

Index: chrome/browser/supervised_user/chromeos/manager_password_service.h
diff --git a/chrome/browser/supervised_user/chromeos/manager_password_service.h b/chrome/browser/supervised_user/chromeos/manager_password_service.h
index 74e7261301636f3357e598bbda0a76e5fbb3040f..77c2574b89ef53b90511b650a5708ae007e96738 100644
--- a/chrome/browser/supervised_user/chromeos/manager_password_service.h
+++ b/chrome/browser/supervised_user/chromeos/manager_password_service.h
@@ -15,6 +15,8 @@
 #include "chrome/browser/supervised_user/supervised_users.h"
 #include "chromeos/login/auth/extended_authenticator.h"
 #include "components/keyed_service/core/keyed_service.h"
+#include "components/user_manager/user_id.h"
+
 
 namespace chromeos {
 
@@ -37,7 +39,7 @@ class ManagerPasswordService
 
   void Shutdown() override;
 
-  void Init(const std::string& user_id,
+  void Init(const user_manager::UserID& user_id,
             SupervisedUserSyncService* user_service,
             SupervisedUserSharedSettingsService* service);
 
@@ -48,11 +50,11 @@ class ManagerPasswordService
   void OnSharedSettingsChange(const std::string& su_id, const std::string& key);
   void GetSupervisedUsersCallback(
       const std::string& sync_su_id,
-      const std::string& user_id,
+      const user_manager::UserID& user_id,
       scoped_ptr<base::DictionaryValue> password_data,
       const base::DictionaryValue* supervised_users);
   void OnAddKeySuccess(const UserContext& master_key_context,
-                       const std::string& user_id,
+                       const user_manager::UserID& user_id,
                        scoped_ptr<base::DictionaryValue> password_data);
   void OnKeyTransformedIfNeeded(const UserContext& master_key_context);
   void OnNewManagerKeySuccess(const UserContext& master_key_context);
@@ -61,7 +63,7 @@ class ManagerPasswordService
 
   // Cached value from Init().
   // User id of currently logged in user, that have supervised users on device.
-  std::string user_id_;
+  user_manager::UserID user_id_;
   SupervisedUserSyncService* user_service_;
   SupervisedUserSharedSettingsService* settings_service_;