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

chrome/browser/chromeos/drive/file_system_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_CHROMEOS_DRIVE_FILE_SYSTEM_INTERFACE_H_
 #define CHROME_BROWSER_CHROMEOS_DRIVE_FILE_SYSTEM_INTERFACE_H_
 
 #include <string>
 #include <vector>
 
 #include "base/memory/scoped_ptr.h"
 #include "chrome/browser/chromeos/drive/drive.pb.h"
 #include "chrome/browser/chromeos/drive/file_system_metadata.h"
 #include "chrome/browser/chromeos/drive/resource_metadata.h"
 #include "google_apis/drive/base_requests.h"
 #include "google_apis/drive/drive_api_requests.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace drive {
 
 class FileSystemObserver;
 
 // Information about search result returned by Search Async callback.
 // This is data needed to create a file system entry that will be used by file
 // browser.
 struct SearchResultInfo {
   SearchResultInfo(const base::FilePath& path, bool is_directory)
       : path(path), is_directory(is_directory) {}
@@ skipping 397 matching lines @@
                                       const MarkMountedCallback& callback) = 0;
 
   // Marks the cached file as unmounted, and runs |callback| upon completion.
   // Note that this method expects that the |cached_file_path| is the path
   // returned by MarkCacheFileAsMounted().
   // |callback| must not be null.
   virtual void MarkCacheFileAsUnmounted(
       const base::FilePath& cache_file_path,
       const FileOperationCallback& callback) = 0;
 
-  // Adds permission as |role| to |email| for the entry at |drive_file_path|.
+  // Adds permission as |role| to |user_id| for the entry at |drive_file_path|.
   // |callback| must not be null.
   virtual 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) = 0;
 
   // Sets the |key| property on the file or directory at |drive_file_path| with
   // the specified |visibility|. If already exists, then it will be overwritten.
   virtual 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) = 0;
 
   // Resets local data.
   virtual void Reset(const FileOperationCallback& callback) = 0;
 
   // Finds a path of an entry (a file or a directory) by |resource_id|.
   virtual void GetPathFromResourceId(const std::string& resource_id,
                                      const GetFilePathCallback& callback) = 0;
 
   // Free drive caches if needed to secure given available spaces. |callback|
   // takes whether given bytes are available or not.
   virtual void FreeDiskSpaceIfNeededFor(
       int64 num_bytes,
       const FreeDiskSpaceCallback& callback) = 0;
 };
 
 }  // namespace drive
 
 #endif  // CHROME_BROWSER_CHROMEOS_DRIVE_FILE_SYSTEM_INTERFACE_H_