Make an OWNERS file for chrome/browser/ui/desktop_ios_promotion

This was requested by pkasting. The owners were specified here:
https://chromium-review.googlesource.com/c/chromium/src/+/1070328#message-2ce2b6f61c3e2e42f9aea96203f51b74b83523da

Change-Id: I44c2605cb0972b6be723c3f880c5bd0dabb75f27
TBR: pkasting@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/1072558
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561708}

chrome/browser/ui/desktop_ios_promotion/OWNERS

+pkl@chromium.org
+justincohen@chromium.org