Fix the History.TopHostsVisitsByRank UMA or add History.TopHostsVisitsByVisitPercentage to get more insight on users browsing habit.

Currently this UMA is populated if precache GCM task has populated the in-memory table host_ranks_.
If chrome is closed and reopened after the precache task is run, host_ranks_ will not be populated.

PrecacheReferrerHostTable is a persistent copy of the top host computation. So this can be used to log this UMA.