Fix serviceworker default registration scope.

A follow up fix for spec update: https://github.com/slightlyoff/ServiceWorker/issues/312.

Review URL: https://codereview.chromium.org/340763002

git-svn-id: svn://svn.chromium.org/blink/trunk@176534 bbb929c8-8fbe-4397-9dbb-9b2b20218538