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

chrome/browser/supervised_user/legacy/supervised_user_refresh_token_fetcher.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_LEGACY_SUPERVISED_USER_REFRESH_TOKEN_FETCHER_H_
 #define CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_REFRESH_TOKEN_FETCHER_H_
 
 #include <string>
 
 #include "base/callback_forward.h"
 #include "base/compiler_specific.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/strings/string16.h"
 
 class GoogleServiceAuthError;
 class OAuth2TokenService;
 
+namespace user_manager {
+class UserID;
+}
+
 namespace net {
 class URLRequestContextGetter;
 }
 
 // This class fetches an OAuth2 refresh token that is tied to a supervised user
 // ID and downscoped to a special scope for Chrome Sync for supervised users.
 // Fetching the token consists of the following steps:
 //   1. Get an access token for the custodian from OAuth2TokenService
 //      (either cached or fetched).
 //   2. Call the IssueToken API to mint a scoped authorization code for a
 //      refresh token for the supervised user from the custodian's access token.
 //   3. Exchange the authorization code for a refresh token for the supervised
 //      user and return it to the caller. The refresh token can only be used to
 //      mint tokens with the special supervised user Sync scope.
 class SupervisedUserRefreshTokenFetcher {
  public:
   typedef base::Callback<void(const GoogleServiceAuthError& /* error */,
                               const std::string& /* refresh_token */)>
       TokenCallback;
 
   static scoped_ptr<SupervisedUserRefreshTokenFetcher> Create(
       OAuth2TokenService* oauth2_token_service,
       const std::string& account_id,
       const std::string& device_id,
       net::URLRequestContextGetter* context);
 
   virtual ~SupervisedUserRefreshTokenFetcher();
 
-  virtual void Start(const std::string& supervised_user_id,
+  virtual void Start(const user_manager::UserID& supervised_user_id,
                      const std::string& device_name,
                      const TokenCallback& callback) = 0;
 };
 
 #endif  // CHROME_BROWSER_SUPERVISED_USER_LEGACY_SUPERVISED_USER_REFRESH_TOKEN_FETCHER_H_