Use new facilities for base::expected: extensions/

Bug: 1424513
Change-Id: I8467b869f27fec85509948030d23e678720e6476
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4755285


Auto-Submit: Peter Kasting <pkasting@chromium.org>
Reviewed-by: David Bertoni <dbertoni@chromium.org>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180468}