[iOS] Add field trial for Hide toolbars

Bug: 507823031
Change-Id: I1947f46d687315cd4e66319b5522c9eb2d78e148
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7793391
Reviewed-by: Scott Yoder <scottyoder@google.com>
Commit-Queue: Aliona Dangla <alionadangla@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1628565}
NOKEYCHECK=True
GitOrigin-RevId: 4c382f2defb9506c06736a78f49d839f38f7e04d