Track support for fetch() keepalive flag (related to navigator.sendBeacon)

[jscinoz]

This feature is already implemented in at least one browser (Safari - release notes).

Spec: https://fetch.spec.whatwg.org/#keep-alive-flag
Chrome status page: https://www.chromestatus.com/feature/5760375567941632
Chromium tracking issue: https://bugs.chromium.org/p/chromium/issues/detail?id=695939
Firefox tracking issue: https://bugzilla.mozilla.org/show_bug.cgi?id=1342484

[toddreifsteck]

Edge was actually first to ship this flag in the Creator's Update ~6 months ago. :-) It is unclear how we should track individual flags on existing APIs. Let me talk to site owners then I'll submit a PR to clean this up.