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

chrome/browser/supervised_user/chromeos/supervised_user_password_service.h

Index: chrome/browser/supervised_user/chromeos/supervised_user_password_service.h
diff --git a/chrome/browser/supervised_user/chromeos/supervised_user_password_service.h b/chrome/browser/supervised_user/chromeos/supervised_user_password_service.h
index b3fe78a432444bdad0ea16cbeb346a8ac385aec6..7c8d4757d771908b4fdf4fdb0584644fa16c6c87 100644
--- a/chrome/browser/supervised_user/chromeos/supervised_user_password_service.h
+++ b/chrome/browser/supervised_user/chromeos/supervised_user_password_service.h
@@ -13,6 +13,7 @@
 #include "chrome/browser/supervised_user/legacy/supervised_user_shared_settings_service.h"
 #include "chrome/browser/supervised_user/supervised_users.h"
 #include "components/keyed_service/core/keyed_service.h"
+#include "components/user_manager/user_id.h"
 
 namespace chromeos {
 
@@ -23,14 +24,14 @@ class SupervisedUserPasswordService : public KeyedService {
 
   void Shutdown() override;
 
-  void Init(const std::string& user_id,
+  void Init(const user_manager::UserID& user_id,
             SupervisedUserSharedSettingsService* service);
  private:
   void OnSharedSettingsChange(const std::string& su_id, const std::string& key);
 
   // Cached value from Init().
   // User id of currently logged in supervised user.
-  std::string user_id_;
+  user_manager::UserID user_id_;
   SupervisedUserSharedSettingsService* settings_service_;
 
   scoped_ptr<SupervisedUserSharedSettingsService::ChangeCallbackList::