base::Time::FromString() and base::Time::FromUTCString() might fail
at runtime on bad input. This adds a WARN_UNUSED_RESULT to their
declarations to ensure that all callers properly handle the result.

Apart from unit-tests, this changes two things:

- DataUseTracker::RemoveExpiredEntriesForPref() will also
  remove any entries with an invalid date string.

  This doesn't change the runtime behaviour though, because
  a failure in base::Time::FromUTCString() would keep the
  local |key_date| variable to 0, which would have been
  considered too old anyway.

- VariationsSeedStore::ImportFirstRunJavaSeed() has
  a new failure mode triggered by an invalid response
  date |FIRST_RUN_SEED_IMPORT_FAIL_INVALID_RESPONSE_DATE|

  It looks like the FirstRunResult enum values are only
  used on Android to report UMA metrics, so ensure that
  the new constant is added at the end of the list to
  avoid generating confusing histograms.

- PexeDownloader::didReceiveResponse() still ignores
  invalid HTTP response 'last-modified' header dates.
  As with RemoveExpiredEntriesForPref() this doesn't
  change the runtime behaviour, but it is hard to see
  whether this is desirable or dangerous.

- ConvertRequestValueToFileInfo() still ignores
  invalid date strings, as mentioned by the comment
  above the base::Time::FromString() line.

BUG=669625

Review-Url: https://codereview.chromium.org/2605293002
Cr-Commit-Position: refs/heads/master@{#443353}