Respect "enabled" param when adding an alert item

The small bug was introduced in https://chromium-review.googlesource.com/c/chromium/src/+/4286550.
It's likely only affecting
ios/chrome/browser/bookmarks/ui_bundled/home/bookmarks_home_view_controller.mm
which is the only place using that version of the API, but it has gone unnoticed since.

Change-Id: I8a2b6057700b5f582c40c18436d6547a5b359955
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6691598
Auto-Submit: Louis Romero <lpromero@google.com>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1480821}