[ServiceWorker] Don't allow registration of the ServiceWorker scope outside the script directory.

As per discussion on https://github.com/slightlyoff/ServiceWorker/issues/468

BUG=423983

Review URL: https://codereview.chromium.org/672383003

git-svn-id: svn://svn.chromium.org/blink/trunk@184336 bbb929c8-8fbe-4397-9dbb-9b2b20218538