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

chrome/browser/chromeos/drive/fake_file_system.h

Index: chrome/browser/chromeos/drive/fake_file_system.h
diff --git a/chrome/browser/chromeos/drive/fake_file_system.h b/chrome/browser/chromeos/drive/fake_file_system.h
index e84d339f928af5bf1f2d371c1bcac472ce22b427..a9e30350085de9f7db667d2f3ad3e174742cd1ed 100644
--- a/chrome/browser/chromeos/drive/fake_file_system.h
+++ b/chrome/browser/chromeos/drive/fake_file_system.h
@@ -22,6 +22,10 @@ class FileResource;
 
 }  // namespace google_apis
 
+namespace user_manager {
+class UserID;
+}
+
 namespace drive {
 
 class DriveServiceInterface;
@@ -115,7 +119,7 @@ class FakeFileSystem : public FileSystemInterface {
   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,