GitLab

- DiscoverFeedService is now in charge of creating and owning a single
instance of FeedMetricsRecorder.
- Add the FeedMetricsRecorder to DiscoverFeedConfig
- Adds engagement recording events

Bug: 1085419, 1127557
Change-Id: Iaf3cdcca850babb54bc8149050a4ebd6c3f8d68c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2406715
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Reviewed-by: Ganggui Tang <gogerald@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806747}