ServiceWorker: Ensure getServiced() resolved with JS array

Just an additional test assertion.

BUG=372749
R=kinuko@chromium.org

Review URL: https://codereview.chromium.org/283673004

git-svn-id: svn://svn.chromium.org/blink/trunk@173962 bbb929c8-8fbe-4397-9dbb-9b2b20218538