[iOS] Add OWNERS for ios/chrome/browser/interstitials

Adding OWNERS file listing people who have recently been working on
the interstitial code and have more familiarity than OWNERS listed
in ios/chrome.

Change-Id: Ifacb6852d7c765f304b47ed2ddbed5fa3a5c6b7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2111355Reviewed-by: Ali Juma <ajuma@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#752037}

ios/chrome/browser/interstitials/OWNERS

+ajuma@chromium.org
+kkhorimoto@chromium.org
+livvielin@chromium.org