Add test for ServiceWorkerRegistration's attributes after register is called

This wasn't systemically tested before. Some tests partially covered it,
which clouds those tests' purpose and makes it unclear what the total
coverage of ServiceWorkerRegistration is.

BUG=396400

Review URL: https://codereview.chromium.org/487203002

git-svn-id: svn://svn.chromium.org/blink/trunk@180619 bbb929c8-8fbe-4397-9dbb-9b2b20218538