ServiceWorker: Add layout test for worker script MIME type.

A worker script with invalid MIME type should not be registered.

BUG=400604

Review URL: https://codereview.chromium.org/448063002

git-svn-id: svn://svn.chromium.org/blink/trunk@181006 bbb929c8-8fbe-4397-9dbb-9b2b20218538