document.lastModified should consider user's local time zone

SPEC: http://www.whatwg.org/specs/web-apps/current-work/#dom-document-lastmodified

Failed test cases will be passed with this CL in
http://w3c-test.org/html/dom/documents/resource-metadata-management/document-lastModified-01.html

Behavior in other browsers.
*)FF: PASS
*)IE: PASS

In addition, this CL consolidates a function that provides local time on given argument.
Subsequently, calculation method for UTC and DST are now same with Date.

TEST=LayoutTests/http/tests/misc/last-modified-parsing.html

Review URL: https://codereview.chromium.org/394903004

git-svn-id: svn://svn.chromium.org/blink/trunk@179277 bbb929c8-8fbe-4397-9dbb-9b2b20218538