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

chrome/browser/chromeos/drive/file_system.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_CHROMEOS_DRIVE_FILE_SYSTEM_H_
 #define CHROME_BROWSER_CHROMEOS_DRIVE_FILE_SYSTEM_H_
 
 #include <string>
 #include <vector>
 
@@ skipping 129 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 63 matching lines @@
   void GetShareUrlAfterGetResourceEntry(const base::FilePath& file_path,
                                         const GURL& embed_origin,
                                         const GetShareUrlCallback& callback,
                                         ResourceEntry* entry,
                                         FileError error);
   void OnGetResourceEntryForGetShareUrl(const GetShareUrlCallback& callback,
                                         google_apis::DriveApiErrorCode status,
                                         const GURL& share_url);
   // Part of AddPermission.
   void AddPermissionAfterGetResourceEntry(
-      const std::string& email,
+      const user_manager::UserID& email,
       google_apis::drive::PermissionRole role,
       const FileOperationCallback& callback,
       ResourceEntry* entry,
       FileError error);
 
   // Part of OnDriveSyncError().
   virtual void OnDriveSyncErrorAfterGetFilePath(
       file_system::DriveSyncErrorType type,
       const base::FilePath* file_path,
       FileError error);
@@ skipping 50 matching lines @@
   // Note: This should remain the last member so it'll be destroyed and
   // invalidate the weak pointers before any other members are destroyed.
   base::WeakPtrFactory<FileSystem> weak_ptr_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(FileSystem);
 };
 
 }  // namespace drive
 
 #endif  // CHROME_BROWSER_CHROMEOS_DRIVE_FILE_SYSTEM_H_