Introduce NavigatorID IDL interface

Introduce NavigatorID IDL interface to avoid duplication and match the
specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/timers.html#navigatorid
http://www.whatwg.org/specs/web-apps/current-work/multipage/timers.html#navigator
http://www.whatwg.org/specs/web-apps/current-work/multipage/workers.html#workernavigator

R=arv@chromium.org, haraken@chromium.org

Review URL: https://codereview.chromium.org/22927015

git-svn-id: svn://svn.chromium.org/blink/trunk@156396 bbb929c8-8fbe-4397-9dbb-9b2b20218538