Add metrics reporting result of ExploreSitesFetcher.

Bug: 867488
Change-Id: Idb8c9aa2ce120f6d8c4d3feb4789b0f81d636818
Reviewed-on: https://chromium-review.googlesource.com/c/1275139
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Justin DeWitt <dewittj@chromium.org>
Commit-Queue: Dmitry Titov <dimich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598914}
6 files changed