[iOS][WidgetKit] Add OWNERS file

Bug: 1138710
Change-Id: Ie9b9e3598d87cfa4cf6bdec46121b06d2e06f885
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495833
Auto-Submit: Javier Ernesto Flores Robles <javierrobles@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820813}

ios/chrome/widget_kit_extension/OWNERS

+javierrobles@chromium.org