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

chrome/browser/drive/drive_service_interface.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_SERVICE_INTERFACE_H_
 #define CHROME_BROWSER_DRIVE_DRIVE_SERVICE_INTERFACE_H_
 
 #include <string>
 
 #include "base/time/time.h"
 #include "google_apis/drive/auth_service_interface.h"
 #include "google_apis/drive/base_requests.h"
 #include "google_apis/drive/drive_api_requests.h"
 #include "google_apis/drive/drive_common_callbacks.h"
 
 namespace base {
 class Time;
 }
 
+namespace user_manager {
+class UserID;
+}
+
 namespace drive {
 
 // Observer interface for DriveServiceInterface.
 class DriveServiceObserver {
  public:
   // Triggered when the service gets ready to send requests.
   virtual void OnReadyToSendRequests() {}
 
   // Called when the refresh token was found to be invalid.
   virtual void OnRefreshTokenInvalid() {}
@@ skipping 417 matching lines @@
 
   // Uninstalls a Drive app with the id |app_id|. |callback| must not be null.
   virtual google_apis::CancelCallback UninstallApp(
       const std::string& app_id,
       const google_apis::EntryActionCallback& callback) = 0;
 
   // Authorizes the account |email| to access |resource_id| as a |role|.
   // |callback| must not be null.
   virtual 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) = 0;
 
   // Starts batch request and returns |BatchRequestConfigurator|.
   virtual scoped_ptr<BatchRequestConfiguratorInterface> StartBatchRequest() = 0;
 };
 
 }  // namespace drive
 
 #endif  // CHROME_BROWSER_DRIVE_DRIVE_SERVICE_INTERFACE_H_