GitLab

Add a Media.Feed.Discover UKM event that is recorded
when we discover a new Media Feed. This is to track
usage of the feature.

BUG=1065014

Change-Id: If0fb31dcf597cc51716c1397904412f199c09524
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2122368
Commit-Queue: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763486}