[ntp] Add //components/search/OWNERS to incognito, guest NTPs

Change-Id: I7e4a3d562a3cbda06950c455554a1da33b6f7e66
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2284092
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785878}

chrome/browser/resources/ntp4/OWNERS

+file://components/search/OWNERS
 estade@chromium.org
 rbyers@chromium.org
 

chrome/browser/ui/webui/ntp/OWNERS

+file://components/search/OWNERS
 estade@chromium.org
 
 per-file app_launcher_handler.*=dmurph@chromium.org