[iOS][Bookmarks storage] Sign-In without using UI

This slightly speed-up tests.

Note that sync being disabled by default on iOS ensure that
`enableSync:YES` is ignored (unless
`kReplaceSyncPromosWithSignInPromos` is disabled), so removing
`enableSync:YES` is also generally valid.

Bug: 325235662
Change-Id: I64bc98312ef1da6b75a19203206e047dd79def48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5296357
Reviewed-by: Tanmoy Mollik <triploblastic@google.com>
Commit-Queue: Arthur Milchior <arthurmilchior@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1262321}