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

chrome/browser/ui/webui/options/supervised_user_import_handler.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_BROWSER_UI_WEBUI_OPTIONS_SUPERVISED_USER_IMPORT_HANDLER_H_
 #define CHROME_BROWSER_UI_WEBUI_OPTIONS_SUPERVISED_USER_IMPORT_HANDLER_H_
 
 #include <string>
 
 #include "base/callback_list.h"
 #include "base/memory/weak_ptr.h"
 #include "base/scoped_observer.h"
 #include "base/strings/string16.h"
 #include "chrome/browser/profiles/profile_info_cache_observer.h"
 #include "chrome/browser/supervised_user/legacy/supervised_user_sync_service_observer.h"
 #include "chrome/browser/ui/webui/options/options_ui.h"
 #include "components/signin/core/browser/signin_error_controller.h"
 
 namespace base {
 class DictionaryValue;
 class ListValue;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 class ProfileInfoCache;
 class SupervisedUserSyncService;
 
 namespace options {
 
 // Handler for the 'import existing supervised user' dialog.
 class SupervisedUserImportHandler : public OptionsPageUIHandler,
                                     public ProfileInfoCacheObserver,
                                     public SupervisedUserSyncServiceObserver,
                                     public SigninErrorController::Observer {
@@ skipping 13 matching lines @@
   void RegisterMessages() override;
 
   // ProfileInfoCacheObserver implementation.
   void OnProfileAdded(const base::FilePath& profile_path) override;
   void OnProfileWillBeRemoved(const base::FilePath& profile_path) override;
   void OnProfileWasRemoved(const base::FilePath& profile_path,
                            const base::string16& profile_name) override;
 
   // SupervisedUserSyncServiceObserver implementation.
   void OnSupervisedUserAcknowledged(
-      const std::string& supervised_user_id) override {}
+      const user_manager::UserID& supervised_user_id) override {}
   void OnSupervisedUsersSyncingStopped() override {}
   void OnSupervisedUsersChanged() override;
 
   // SigninErrorController::Observer implementation.
   void OnErrorChanged() override;
 
   // Clears the cached list of supervised users and fetches the new list of
   // supervised users.
   void FetchSupervisedUsers();
 
@@ skipping 39 matching lines @@
   bool removed_profile_is_supervised_;
 
   base::WeakPtrFactory<SupervisedUserImportHandler> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(SupervisedUserImportHandler);
 };
 
 }  // namespace options
 
 #endif  // CHROME_BROWSER_UI_WEBUI_OPTIONS_SUPERVISED_USER_IMPORT_HANDLER_H_