Add ServiceWorkerNavigationPreloadTest for resolving navigationPreload with null.

At the discussion in https://crrev.com/2416843002/#msg62, I decided not to use
the nullable promise type. So when navigation preload is not enabled,
FetchEvent.navigationPreload must return a promise which will be resolved with
null.

This CL adds a test for this behavior.

BUG=649558

Review-Url: https://codereview.chromium.org/2446083002
Cr-Commit-Position: refs/heads/master@{#427745}
1 file changed