[history] Add `is_known_to_sync` column to the `visits` table

Adds a new boolean `is_known_to_sync` column to the `visits` table so
we can track for which visits we can send URL-keyed metadata requests.

This CL is ONLY the CL to do the SQL migration, and doesn't add logic
to actually set it. So after this CL, all the flags will be left as
FALSE, which is actually fine, because that's the safe state.

(False negatives are totally fine, because that's today's status quo,
false positives are not okay.)

Followups:
 1. Set this field to true for all local visits created while Sync is
    is enabled for the user.
 2. Set this field to true for incoming  all remote visits as they are
    created by Sync.
 3. Clearing out all these bits when Sync is explicitly disabled.
 4. Adding a (delayed) check on Chrome startup to clear out this bit
    in case Sync got disabled and we missed the above notification.

Change-Id: If503288b6d741cf2fe6be1be9a52713ef3b140b4
Bug: 1256979
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3859672


Commit-Queue: Tommy Li <tommycli@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1042612}