This fixes the test being dependent on the system time, and undefined
behaviour resulting from negative timestamps in positive-offset
timezones.

BUG=581895

Review URL: https://codereview.chromium.org/1640973002

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@298438 0039d316-1c4b-4281-b951-d872f2087c98