Implement ServiceWorkerGlobalScope 'scope' attribute [Tests]

New tests only. Originally reviewed as part of crrev.com/224863002
but split out for a 3-part landing (blink, chromium, tests).

TBR=kinuko@chromium.org,michaeln@chromium.org

Review URL: https://codereview.chromium.org/238933008

git-svn-id: svn://svn.chromium.org/blink/trunk@171722 bbb929c8-8fbe-4397-9dbb-9b2b20218538