Revert of Make dates default to the local timezone if none specified  (https://codereview.chromium.org/1229903004/)

Even though the change is ES6 spec compliant, we decided to revert
to be consistent with other browsers and work on fixing the spec.

Original issue's description:
> Make dates default to the local timezone if none specified
>
> 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

> Committed: https://crrev.com/f06754a8e1d305a43560705f6c167d85d40e602d
> Cr-Commit-Position: refs/heads/master@{#29854}

BUG=chromium:543320,chromium:539813
LOG=NO

Review URL: https://codereview.chromium.org/1403153003

Cr-Commit-Position: refs/heads/master@{#31295}