In ES2015, section 20.3.1.15, TimeClip (an internal algorihtm which is
called when normalizing Date representations) is specified to add 0
to its result, which converts -0 into +0. This patch adds that conversion
to the Date code. It is verified by the test262 test
built-ins/Date/TimeClip_negative_zero

R=adamk

Review URL: https://codereview.chromium.org/1387293002

Cr-Commit-Position: refs/heads/master@{#31159}