Fix computation of UTC time from local time at DST change points.

This also reverts r23606, which was an incorrect fix.

BUG=v8:3116,chromium:417640,chromium:415424
LOG=Y
TEST=mjsunit/regress/regress-3116.js
R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/639383002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24499 ce2b1a6d-e550-0410-aec6-3dcde31c8c00