Add OWNERS file for ad_metrics/ directory

Adds new owners file specifically for the AdsPLMO directory.

Change-Id: I01028ae59ee4b27211f8deb189199c0ae05fd65c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359587Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: John Delaney <johnidel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799179}

chrome/browser/page_load_metrics/observers/ad_metrics/OWNERS

+johnidel@chromium.org
+ericrobinson@chromium.org
+jkarlin@chromium.org
+
+# COMPONENT: UI>Browser>AdFilter