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

chrome/browser/chromeos/drive/fake_file_system.cc

 // Copyright (c) 2013 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.
 
 #include "chrome/browser/chromeos/drive/fake_file_system.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/callback.h"
 #include "base/files/file_path.h"
 #include "base/files/file_util.h"
 #include "base/logging.h"
 #include "chrome/browser/chromeos/drive/drive.pb.h"
 #include "chrome/browser/chromeos/drive/file_errors.h"
 #include "chrome/browser/chromeos/drive/file_system_util.h"
 #include "chrome/browser/chromeos/drive/resource_entry_conversion.h"
 #include "chrome/browser/drive/drive_service_interface.h"
+#include "components/user_manager/user_id.h"
 #include "content/public/browser/browser_thread.h"
 #include "google_apis/drive/drive_api_parser.h"
 
 namespace drive {
 namespace test_util {
 
 using content::BrowserThread;
 
 FakeFileSystem::FakeFileSystem(DriveServiceInterface* drive_service)
     : drive_service_(drive_service),
@@ skipping 185 matching lines @@
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 }
 
 void FakeFileSystem::MarkCacheFileAsUnmounted(
     const base::FilePath& cache_file_path,
     const FileOperationCallback& callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 }
 
 void FakeFileSystem::AddPermission(const base::FilePath& drive_file_path,
-                                   const std::string& email,
+                                   const user_manager::UserID& user_id,
                                    google_apis::drive::PermissionRole role,
                                    const FileOperationCallback& callback) {
   DCHECK_CURRENTLY_ON(BrowserThread::UI);
 }
 
 void FakeFileSystem::SetProperty(
     const base::FilePath& drive_file_path,
     google_apis::drive::Property::Visibility visibility,
     const std::string& key,
     const std::string& value,
@@ skipping 172 matching lines @@
       callback.Run(FILE_ERROR_OK, entry.Pass());
       return;
     }
   }
 
   callback.Run(FILE_ERROR_NOT_FOUND, scoped_ptr<ResourceEntry>());
 }
 
 }  // namespace test_util
 }  // namespace drive