Replace local-midnight timestamps with UTC midnight throughout
`SerpMetrics` and `SerpMetricsTimePeriodStorage` so daily report
windows are always exactly 24 hours and unaffected by DST
transitions. Add a `kLastReportedAt` preference to store the
timestamp of each successful daily ping, giving
`GetStartOfStalePeriod` a UTC-aligned starting point. Existing
profiles that have no `kLastReportedAt` value continue migrating
from the legacy local-date string in `kLastCheckYMD` until they
send their first report under the new scheme. Remove the
DST-specific test files since UTC boundaries need no DST correction.
Aleksei Seren
d5f5a2ecc4