Use new facilities for base::expected: storage/browser/file_system/

Bug: 1424513
Change-Id: Ib24afb9aa58a86f46db0054bf6b99fd7d238d052
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4755212


Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Austin Sullivan <asully@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180445}