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

chrome/browser/chromeos/drive/file_system_interface.h

Index: chrome/browser/chromeos/drive/file_system_interface.h
diff --git a/chrome/browser/chromeos/drive/file_system_interface.h b/chrome/browser/chromeos/drive/file_system_interface.h
index ebe632321ef26d5551538e85ab513d85bb246373..f4c6cebfc42c752e43c1ea8e15c0ddbf26782b58 100644
--- a/chrome/browser/chromeos/drive/file_system_interface.h
+++ b/chrome/browser/chromeos/drive/file_system_interface.h
@@ -15,6 +15,10 @@
 #include "google_apis/drive/base_requests.h"
 #include "google_apis/drive/drive_api_requests.h"
 
+namespace user_manager {
+class UserID;
+}
+
 namespace drive {
 
 class FileSystemObserver;
@@ -432,10 +436,10 @@ class FileSystemInterface {
       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;