[omnibox] Append extra header to @google built-in keyword navigations

Bug: 1521556

Change-Id: I1869c5ef6b0373e83f826b7f0547f1506ab8da99
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5287249


Reviewed-by: Angela Yoeurng <yoangela@chromium.org>
Commit-Queue: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1261192}