Owner change for chrome/browser/downgrade

Add Greg and remove Georges.

Change-Id: Ia1d75e45a5083d32ea7198394c34f274dc765a9b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1787864Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Owen Min <zmin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693747}

chrome/browser/downgrade/OWNERS

-georgesak@chromium.org
+grt@chromium.org
 zmin@chromium.org
 # COMPONENT: Enterprise