[css-layout-api] Fix tests OWNERS file.

In:
https://chromium-review.googlesource.com/c/chromium/src/+/1761067

We moved all the custom layout files to their new NG directory, however
we had this link still pointing at a (now non-existent) OWNERS file.

NOPRESUBMIT=true

Change-Id: I1ba9651947a189a82ce493c3e09585ec2d188964
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1761774
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Auto-Submit: Ian Kilpatrick <ikilpatrick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#688653}

third_party/blink/web_tests/external/wpt/css/css-layout-api/OWNERS

 # TEAM: layout-dev@chromium.org
 # COMPONENT: Blink>Layout
 # WPT-NOTIFY: true
-file://third_party/blink/renderer/core/layout/custom/OWNERS
+file://third_party/blink/renderer/core/layout/ng/custom/OWNERS