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

chrome/browser/chromeos/drive/fake_file_system.h

 // 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.
 
 #ifndef CHROME_BROWSER_CHROMEOS_DRIVE_FAKE_FILE_SYSTEM_H_
 #define CHROME_BROWSER_CHROMEOS_DRIVE_FAKE_FILE_SYSTEM_H_
 
 #include <string>
 
 #include "base/basictypes.h"
 #include "base/callback_forward.h"
 #include "base/files/scoped_temp_dir.h"
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chromeos/drive/file_errors.h"
 #include "chrome/browser/chromeos/drive/file_system_interface.h"
 #include "google_apis/drive/drive_api_error_codes.h"
 
 namespace google_apis {
 
 class AboutResource;
 class FileResource;
 
 }  // namespace google_apis
 
+namespace user_manager {
+class UserID;
+}
+
 namespace drive {
 
 class DriveServiceInterface;
 class FileSystemObserver;
 class ResourceEntry;
 
 namespace test_util {
 
 // This class implements a fake FileSystem which acts like a real Drive
 // file system with FakeDriveService, for testing purpose.
@@ skipping 73 matching lines @@
   void GetAvailableSpace(const GetAvailableSpaceCallback& callback) override;
   void GetShareUrl(const base::FilePath& file_path,
                    const GURL& embed_origin,
                    const GetShareUrlCallback& callback) override;
   void GetMetadata(const GetFilesystemMetadataCallback& callback) override;
   void MarkCacheFileAsMounted(const base::FilePath& drive_file_path,
                               const MarkMountedCallback& callback) override;
   void MarkCacheFileAsUnmounted(const base::FilePath& cache_file_path,
                                 const FileOperationCallback& callback) override;
   void AddPermission(const base::FilePath& drive_file_path,
-                     const std::string& email,
+                     const user_manager::UserID& email,
                      google_apis::drive::PermissionRole role,
                      const FileOperationCallback& callback) override;
   void SetProperty(const base::FilePath& drive_file_path,
                    google_apis::drive::Property::Visibility visibility,
                    const std::string& key,
                    const std::string& value,
                    const FileOperationCallback& callback) override;
   void Reset(const FileOperationCallback& callback) override;
   void GetPathFromResourceId(const std::string& resource_id,
                              const GetFilePathCallback& callback) override;
@@ skipping 57 matching lines @@
   // invalidate the weak pointers before any other members are destroyed.
   base::WeakPtrFactory<FakeFileSystem> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(FakeFileSystem);
 };
 
 }  // namespace test_util
 }  // namespace drive
 
 #endif  // CHROME_BROWSER_CHROMEOS_DRIVE_FAKE_FILE_SYSTEM_H_