Add OWNERS for external/wpt/picture-in-picture

so that FindIt can route the bugs correctly.

Change-Id: If722f7ecd78b9eb918d2a79253e9b0d4df6be06e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2398723
Auto-Submit: Robert Ma <robertma@chromium.org>
Commit-Queue: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807296}

third_party/blink/web_tests/external/wpt/picture-in-picture/OWNERS

+# COMPONENT: Blink>Media>PictureInPicture