Fix the tz detection code.

Due to caching, the 2nd timezone change is not detected properly.

Upstream bug: http://bugs.icu-project.org/trac/ticket/11623

BUG=406382
TEST=manual. Call detectHostTimeZone() every time the OS tz is changed on Linux and check if it returns the correct present timezone.
R=mark@chromium.org

Review URL: https://codereview.chromium.org/1058353002
5 files changed