Add UMA for ExploreSites NTP section - opening and loading.

Adds Android.ExploreSitesNTP.Opened user action and
ExploreSites.NTPLoadingCatalogFromNetwork histogram to record if the local catalog
was used or we had to initiate network request to fetch one.

Bug: 867488
Change-Id: Ie5c3593c902dd20594bce9a982ef4b5b9573d449
Reviewed-on: https://chromium-review.googlesource.com/c/1269443
Commit-Queue: Dmitry Titov <dimich@chromium.org>
Reviewed-by: Cathy Li <chili@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597760}
3 files changed