List support for the window.fetch keepalive option

[bonitao]

Details: https://www.chromestatus.com/features/5760375567941632

[scomo]

The fact that firefox latest does not support keepalive needs to be on the fetch status page, possibly sub feature or known issue. https://bugzilla.mozilla.org/show_bug.cgi?id=1342484