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

chrome/browser/chromeos/drive/fake_file_system.cc

Index: chrome/browser/chromeos/drive/fake_file_system.cc
diff --git a/chrome/browser/chromeos/drive/fake_file_system.cc b/chrome/browser/chromeos/drive/fake_file_system.cc
index 3017d8cf08b0b3ec0ece685f46b2edf23f5c9cad..5bdbc6593cbc490b3aceb4c591e9f26f0febfbf0 100644
--- a/chrome/browser/chromeos/drive/fake_file_system.cc
+++ b/chrome/browser/chromeos/drive/fake_file_system.cc
@@ -15,6 +15,7 @@
 #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"
 
@@ -220,7 +221,7 @@ void FakeFileSystem::MarkCacheFileAsUnmounted(
 }
 
 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);