[ios/web] Move SlimNav VisibleItem trust check to LastCommitted.

As a followup to http://crrev.com/c/1669577, move the trust check to
LastCommitted so all WebState APIs can return a trusted URL rather than
just VisibleItem.

BUG: 971740
Change-Id: Ia912007c6bfc65efa0443a07b072d9ed8075cedd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1673566
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672146}
8 files changed