Make the function return Maybe<std::string> to propagate error.

Bug: chromium:917151
Change-Id: I4330b0c54c122b1eddd3b37e21e0f387cb5d803a
Reviewed-on: https://chromium-review.googlesource.com/c/1392205Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Frank Tang <ftang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58535}