[ios] Add thegreenfrog@ as fullscreen owner

Change-Id: I22d295b2e3f014b2d279ef6729e405399dd15349
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2227394Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Chris Lu <thegreenfrog@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774773}

ios/chrome/browser/ui/fullscreen/OWNERS

-kkhorimoto@chromium.org
+thegreenfrog@chromium.org
 sczs@chromium.org