[iOS] Add ios/components/security_interstitials

Use the same OWNERS as ios/chrome/browser/interstitials, since that
code will be moved in to this component.

Bug: 1064382
Change-Id: I8a1aaee104b951edb052b5ce202e70cfd9654ff7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2133208Reviewed-by: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Livvie Lin <livvielin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756035}

ios/components/security_interstitials/OWNERS

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