In locations that observe daylight saving time, this method now probes
different times to determine the offset when DST observance is (or is
not) in effect. This replaces a hard-coded one-hour offset, accounting
for Lord Howe Island, Australia, which is at a 30-minute offset.
TEST=snapshot_test SystemSnapshotMacTest.TimeZone
R=rsesek@chromium.org
Review URL: https://codereview.chromium.org/626093005