Use the existing {base::Optional} instead of the extra {MaybeBoolFlag}
struct. This makes writing to a maybe-flag simpler because you just
write a boolean value and that automatically initializes the optional.

R=cbruni@chromium.org

Bug: v8:12887
Change-Id: I940d20286d65ba4355dc04b4b6068a306706f295
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3686412Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80915}