Service Worker unregistration should not fail if not found (test update)

Follow-on to crrev.com/247773004 on the Chromium side. Updated
expectations and cases for unregister() not rejecting if the
registration was not found.

BUG=365926

Review URL: https://codereview.chromium.org/248103005

git-svn-id: svn://svn.chromium.org/blink/trunk@172423 bbb929c8-8fbe-4397-9dbb-9b2b20218538