Update description of History.MonthlyHostCount to include ref to a similar metric.

The new metric ExploreSites.MonthlyHostCount counts the same thing but is reported
on elapsed-time basis instead of on on-startup basis to help avoid bias toward
more active users who start Chrome more often.
Also updated 'units' for related histograms.

Bug: 867488
Change-Id: I22915ed143af031dadb74aaee5d9b6260726f1af
Reviewed-on: https://chromium-review.googlesource.com/c/1265496
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Dmitry Titov <dimich@chromium.org>
Cr-Commit-Position: refs/heads/master@{#597294}
1 file changed