Introduce NavigatorOnLine interface

Introduce NavigatorOnLine interface and have Navigator and WorkerNavigator
implement it to avoid duplication and match the specification:
http://www.whatwg.org/specs/web-apps/current-work/multipage/offline.html#navigatoronline
http://www.whatwg.org/specs/web-apps/current-work/multipage/workers.html#workernavigator
http://www.whatwg.org/specs/web-apps/current-work/multipage/timers.html#navigator

R=haraken@chromium.org

Review URL: https://codereview.chromium.org/23308006

git-svn-id: svn://svn.chromium.org/blink/trunk@156392 bbb929c8-8fbe-4397-9dbb-9b2b20218538