In ES5, dates were supposed to default to UTC if no timezone was specified. However, this changed in ES6, which specified that dates should be in the local timezone if no timezone was specified. This CL updates our behavior to match that part of the ES6 spec.

BUG=chromium:391730, v8:4242
LOG=Y

Review URL: https://codereview.chromium.org/1229903004

Cr-Commit-Position: refs/heads/master@{#29854}