[ServiceWorker] LayoutTest for event.request.referrer.

This change depends on http://crrev.com/329853012 and http://crrev.com/373713003

BUG=373120

Review URL: https://codereview.chromium.org/373713004

git-svn-id: svn://svn.chromium.org/blink/trunk@177822 bbb929c8-8fbe-4397-9dbb-9b2b20218538