Add HistoryStatisticsReporter for upcoming ExploreSites feature.

It should compute and report monthly visited host count approximatley every week.
It also only runs once per Chrome session and after a delay to avoid slowing down startup.
The average cost of computation is less than 10ms and most of it is on background thread, however
this is still enough to implement 'sparse' way of querying.

Bug: 867488
Change-Id: I000ef4224f87d4d98c0a73519ce9648feb158df7
Reviewed-on: https://chromium-review.googlesource.com/c/1260494
Commit-Queue: Dmitry Titov <dimich@chromium.org>
Reviewed-by: Dominic Battré <battre@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597217}
11 files changed