Implement ServiceWorkerGlobalScope 'scope' attribute [Blink]

Plumb the scope - a URL/wildcard pattern used to register the Service
Worker and control its, er, scope of influence - back to the worker
script instance.

Review URL: https://codereview.chromium.org/224863002

git-svn-id: svn://svn.chromium.org/blink/trunk@171510 bbb929c8-8fbe-4397-9dbb-9b2b20218538