Expose WorkerNavigator interface to web workers

According to the specification, the WorkerNavigator interface is not a
[NoInterfaceObject] and it should be exposed to JavaScript if the JavaScript
global environment is a worker environment:
http://www.whatwg.org/specs/web-apps/current-work/multipage/workers.html#workernavigator

BUG=276160
R=haraken@chromium.org, jochen@chromium.org

Review URL: https://codereview.chromium.org/23163011

git-svn-id: svn://svn.chromium.org/blink/trunk@156395 bbb929c8-8fbe-4397-9dbb-9b2b20218538