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

chrome/browser/supervised_user/supervised_user_service.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_SUPERVISED_USER_SUPERVISED_USER_SERVICE_H_
 #define CHROME_BROWSER_SUPERVISED_USER_SUPERVISED_USER_SERVICE_H_
 
 #include <map>
 #include <string>
 #include <vector>
@@ skipping 40 matching lines @@
 }
 
 namespace extensions {
 class ExtensionRegistry;
 }
 
 namespace user_prefs {
 class PrefRegistrySyncable;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 // This class handles all the information related to a given supervised profile
 // (e.g. the installed content packs, the default URL filtering behavior, or
 // manual whitelist/blacklist overrides).
 class SupervisedUserService : public KeyedService,
 #if defined(ENABLE_EXTENSIONS)
                               public extensions::ManagementPolicy::Provider,
 #endif
                               public SyncTypePreferenceProvider,
                               public sync_driver::SyncServiceObserver,
                               public chrome::BrowserListObserver,
@@ skipping 66 matching lines @@
   void InitSync(const std::string& refresh_token);
 
   // Convenience method that registers this supervised user using
   // |registration_utility| and initializes sync with the returned token.
   // The |callback| will be called when registration is complete,
   // whether it succeeded or not -- unless registration was cancelled manually,
   // in which case the callback will be ignored.
   void RegisterAndInitSync(
       SupervisedUserRegistrationUtility* registration_utility,
       Profile* custodian_profile,
-      const std::string& supervised_user_id,
+      const user_manager::UserID& supervised_user_id,
       const AuthErrorCallback& callback);
 
   void AddNavigationBlockedCallback(const NavigationBlockedCallback& callback);
   void DidBlockNavigation(content::WebContents* web_contents);
 
   void AddObserver(SupervisedUserServiceObserver* observer);
   void RemoveObserver(SupervisedUserServiceObserver* observer);
 
   void AddPermissionRequestCreator(
       scoped_ptr<PermissionRequestCreator> creator);
@@ skipping 192 matching lines @@
 
   // Used to create permission requests.
   ScopedVector<PermissionRequestCreator> permissions_creators_;
 
   ObserverList<SupervisedUserServiceObserver> observer_list_;
 
   base::WeakPtrFactory<SupervisedUserService> weak_ptr_factory_;
 };
 
 #endif  // CHROME_BROWSER_SUPERVISED_USER_SUPERVISED_USER_SERVICE_H_