[iOS] Add ios/chrome/browser/ui/overlays/OWNERS

This CL adds kkhorimoto@ (author of code) and michaeldo@ (reviewer) as
OWNERS for OverlayService.  Both OWNERS have been committers for over
6 months, and already own the model layer of the code at
ios/chrome/browser/overlays/.

Bug: 941745
Change-Id: Id6c6ddca8109963b8459fd41e122cae82a7438b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1635353
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Auto-Submit: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#664354}

ios/chrome/browser/ui/overlays/OWNERS

+kkhorimoto@chromium.org
+michaeldo@chromium.org
+
+# TEAM: ios-directory-owners@chromium.org
+# OS: iOS