[ServiceWorker] Refactor fetch.html layouttest to use service_worker_test()

In this change, I remove the test of fetch('http://www.example.com/foo') because it is insufficient.

TEST=LayoutTests/http/tests/serviceworker/fetch.html
BUG=

Review URL: https://codereview.chromium.org/532163003

git-svn-id: svn://svn.chromium.org/blink/trunk@181418 bbb929c8-8fbe-4397-9dbb-9b2b20218538