[ServiceWorker] Add test for FetchEvent's body with a local file.

This test uses eventSender.beginDragWithFiles() in JS to upload the local file.
So we have to locate the files under LayoutTests/http/tests/local/.

BUG=402387

Review URL: https://codereview.chromium.org/491203002

git-svn-id: svn://svn.chromium.org/blink/trunk@180945 bbb929c8-8fbe-4397-9dbb-9b2b20218538