[omnibox] Prototype showing MVT suggestions in zps on web/srp.

kMostVisitedSitesOnFocus is assumed to be a desktop-only flag.

Parameterizes how many search/url suggest should be shown for LE +
feature iterations where there may be different backfills and suggestion
types.

Screencast: screencast/cast/NDU1MTE5Mjg5MjkzMjA5NnxhMTk3NzExZC1jYg

Change-Id: Id41f2af70483dcd0bcbe1ce11fa1e45523cf7382
Bug: 397762064
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6297883
Commit-Queue: Nihar Majmudar <niharm@google.com>
Reviewed-by: Orin Jaworski <orinj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1427415}