Revert 160785 "Remove the DateExtension"

Broke Linux ChromiumOS Builder:
chrome_render_process_observer.cc:356:5: error: 'enableDateExtension' is not a member of 'WebKit::WebRuntimeFeatures'

> Remove the DateExtension
> 
> The DateExtension tried to kill unload handlers that delayed unload by
> sleeping. However, if a page does this on purpose, it is easy to work
> around this, plus running v8::Extensions slows down context creation.
> 
> BUG=299746
> R=abarth@chromium.org
> 
> Review URL: https://codereview.chromium.org/25001004

TBR=jochen@chromium.org

Review URL: https://codereview.chromium.org/50133003

git-svn-id: svn://svn.chromium.org/blink/trunk@160786 bbb929c8-8fbe-4397-9dbb-9b2b20218538