[iOS] Create OWNERS file for fullscreen directory.

This previously existed, but was mistakenly removed in
crrev.com/c/648076.

Bug: none
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I09759933d8717d55de1343852827ab3289a4bee0
Reviewed-on: https://chromium-review.googlesource.com/830136
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524444}

ios/chrome/browser/ui/fullscreen/OWNERS

+kkhorimoto@chromium.org
+sczs@chromium.org