[ServiceWorker] Send the body of Request in fetch() API.


TEST=LayoutTests/http/tests/serviceworker/resources/request-worker.js
BUG=402387

Review URL: https://codereview.chromium.org/533123002

git-svn-id: svn://svn.chromium.org/blink/trunk@181436 bbb929c8-8fbe-4397-9dbb-9b2b20218538