Improve SystemSnapshotMac::TimeZone()’s computation of UTC offsets

Improve SystemSnapshotMac::TimeZone()’s computation of UTC offsets.

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

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/0ff46a4e602cd42de5cc5e9da21e1b69387a9f16

[Mark Mentovai, 2014-10-03 21:05:42]

mark@chromium.org changed reviewers:
+ rsesek@chromium.org

[Mark Mentovai, 2014-10-03 21:05:43]

This is a more correct way to handle things, in light of
https://codereview.chromium.org/626013002/diff/1/snapshot/system_snapshot_mac....

[Robert Sesek, 2014-10-03 22:36:42]

LGTM. Timezones are hard.

https://codereview.chromium.org/626093005/diff/1/snapshot/system_snapshot_mac.cc
File snapshot/system_snapshot_mac.cc (right):

https://codereview.chromium.org/626093005/diff/1/snapshot/system_snapshot_mac...
snapshot/system_snapshot_mac.cc:360: // saving time in effect. mktime() accepts
values of tm_mon that are
Pinkerton will be happy that I didn't have to comment "savings" -> "saving".

[Mark Mentovai, 2014-10-03 22:40:29]

Committed patchset #1 (id:1) manually as
0ff46a4e602cd42de5cc5e9da21e1b69387a9f16 (presubmit successful).