Add tentative WPT tests for stale while revalidate handling.

Add test to ensure that handling the fetch doesn't trigger stale
while revalidate loading.

Add test to ensure that scripts loaded trigger a stale while revalidate
cache hit and resource timing entries are correct.

The PR for the spec changes is here:
https://github.com/whatwg/fetch/pull/853

BUG=348877

Change-Id: Ib07b98d0d2595b6b99857161f830343bf7516518
Reviewed-on: https://chromium-review.googlesource.com/c/1383297
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: Ben Kelly <wanderview@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625228}
5 files changed