[iOS][AutofillAI] Add field trial entry for Autofill AI

Bug: 498500266
Change-Id: I7b525ec47b222c8d67d10d3da8959ce69712dc0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7719003
Reviewed-by: Norge Vizcay <vizcay@google.com>
Reviewed-by: Alexis Hétu <sugoi@chromium.org>
Commit-Queue: Vidhan Jain <vidhanj@google.com>
Cr-Commit-Position: refs/heads/main@{#1608489}
NOKEYCHECK=True
GitOrigin-RevId: 771bc792ed7d6bb2703bbbdb6b0c02277d9bc8de