[ios] Add OWNERS file to NewTabPageTabHelper.

justincohen@ is an author of all code in
ios/chrome/browser/ntp

Change-Id: I5888fc83e7fa5aabc3187d2c12456adce9ab1da8
Reviewed-on: https://chromium-review.googlesource.com/c/1308994Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#604282}

ios/chrome/browser/ntp/OWNERS

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