Implement ServiceWorkerObject.{url,scope}

BUG=373160
TEST=http/tests/serviceworker/serviceworkerobject-scope.html
TEST=http/tests/serviceworker/current-on-load.html

Review URL: https://codereview.chromium.org/286543005

git-svn-id: svn://svn.chromium.org/blink/trunk@174037 bbb929c8-8fbe-4397-9dbb-9b2b20218538