Correct moves of const std::optional<> contents.

Use a variety of techniques to avoid unintentional copies.

-- remove stray consts that block move.
-- pass by const-ref where reasonable.
-- remove optional<> if value always present.
-- convert one else to early return to prove moved lifetime ends.

This CL was uploaded by git cl split.

R=ayui@chromium.org, clamy@chromium.org, hayato@chromium.org, toyoshim@chromium.org

Bug: 369952985
Change-Id: Ie66a7e6d4edc198f694d80e28ddd2f08b547cdfd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5906790
Commit-Queue: Camille Lamy <clamy@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Hayato Ito <hayato@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1364264}