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

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

Index: chrome/browser/ui/webui/options/create_profile_handler.h
diff --git a/chrome/browser/ui/webui/options/create_profile_handler.h b/chrome/browser/ui/webui/options/create_profile_handler.h
index 6ee91ba07b6959eade3d60287905f834eb026a34..4b8d6e4f6f2e7b3fdcd74804b81eed6feb572dec 100644
--- a/chrome/browser/ui/webui/options/create_profile_handler.h
+++ b/chrome/browser/ui/webui/options/create_profile_handler.h
@@ -19,6 +19,10 @@ class DictionaryValue;
 class ListValue;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 #if defined(ENABLE_SUPERVISED_USERS)
 class SupervisedUserRegistrationUtility;
 #endif
@@ -75,13 +79,13 @@ class CreateProfileHandler: public OptionsPageUIHandler {
   // as the final task after a new profile has been created.
   void OnProfileCreated(bool create_shortcut,
                         chrome::HostDesktopType desktop_type,
-                        const std::string& supervised_user_id,
+                        const user_manager::UserID& supervised_user_id,
                         Profile* profile,
                         Profile::CreateStatus status);
 
   void HandleProfileCreationSuccess(bool create_shortcut,
                                     chrome::HostDesktopType desktop_type,
-                                    const std::string& supervised_user_id,
+                                    const user_manager::UserID& supervised_user_id,
                                     Profile* profile);
 
   // Creates desktop shortcut and updates the UI to indicate success
@@ -125,7 +129,7 @@ class CreateProfileHandler: public OptionsPageUIHandler {
   // sets |profile_creation_type_| if necessary, and returns true if the
   // supervised user id specified in |args| are valid.
   bool ProcessSupervisedCreateProfileArgs(const base::ListValue* args,
-                                          std::string* supervised_user_id);
+                                          user_manager::UserID* supervised_user_id);
 
   // Cancels creation of a supervised-user profile currently in progress, as
   // indicated by profile_path_being_created_, removing the object and files
@@ -143,7 +147,7 @@ class CreateProfileHandler: public OptionsPageUIHandler {
   // with the management server.
   void RegisterSupervisedUser(bool create_shortcut,
                               chrome::HostDesktopType desktop_type,
-                              const std::string& managed_user_id,
+                              const user_manager::UserID& managed_user_id,
                               Profile* new_profile);
 
   // Called back with the result of the supervised user registration.
@@ -158,7 +162,7 @@ class CreateProfileHandler: public OptionsPageUIHandler {
       GoogleServiceAuthError::State error_state);
 
   bool IsValidExistingSupervisedUserId(
-      const std::string& existing_supervised_user_id) const;
+      const user_manager::UserID& existing_supervised_user_id) const;
 
   scoped_ptr<SupervisedUserRegistrationUtility>
       supervised_user_registration_utility_;