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

chrome/browser/drive/drive_api_service.h

 // Copyright (c) 2012 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_DRIVE_DRIVE_API_SERVICE_H_
 #define CHROME_BROWSER_DRIVE_DRIVE_API_SERVICE_H_
 
 #include <string>
 
 #include "base/memory/ref_counted.h"
@@ skipping 217 matching lines @@
       const google_apis::ProgressCallback& progress_callback) override;
   google_apis::CancelCallback AuthorizeApp(
       const std::string& resource_id,
       const std::string& app_id,
       const google_apis::AuthorizeAppCallback& callback) override;
   google_apis::CancelCallback UninstallApp(
       const std::string& app_id,
       const google_apis::EntryActionCallback& callback) override;
   google_apis::CancelCallback AddPermission(
       const std::string& resource_id,
-      const std::string& email,
+      const user_manager::UserID& user_id,
       google_apis::drive::PermissionRole role,
       const google_apis::EntryActionCallback& callback) override;
   scoped_ptr<BatchRequestConfiguratorInterface> StartBatchRequest() override;
 
  private:
   // AuthServiceObserver override.
   void OnOAuth2RefreshTokenChanged() override;
 
   // The class is expected to run on UI thread.
   base::ThreadChecker thread_checker_;
 
   OAuth2TokenService* oauth2_token_service_;
   scoped_refptr<net::URLRequestContextGetter> url_request_context_getter_;
   scoped_refptr<base::SequencedTaskRunner> blocking_task_runner_;
   scoped_ptr<google_apis::RequestSender> sender_;
   ObserverList<DriveServiceObserver> observers_;
   google_apis::DriveApiUrlGenerator url_generator_;
   const std::string custom_user_agent_;
 
   DISALLOW_COPY_AND_ASSIGN(DriveAPIService);
 };
 
 }  // namespace drive
 
 #endif  // CHROME_BROWSER_DRIVE_DRIVE_API_SERVICE_H_