[ios] Convert absl::optional<T> to std::optional<T>

Since https://crrev.com/c/4469528, absl::optional<T> is an alias for
std::optional<T>, so convert uses of the old type to the new one as
it is easier to use (no needs to have public_deps on absl, shorter
path to the header, ...).

This CL converts all uses in //ios/web and below.

Bug: none
Change-Id: I7388d1155304fc88bf30f11d97a3c150ec729a37
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5008129
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Auto-Submit: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1221755}