fetchLater support in Safari

[nicjansma]

Tracking issue for fetchLater() support in Safari.

[tunetheweb]

Webkit tracking bug: https://bugs.webkit.org/show_bug.cgi?id=284347

[nicjansma]

Position: Positive

https://webkit.org/standards-positions/#position-85

[mangs]

WHATWG fetchLater PR: whatwg/fetch#1647

Links to a discussion showing positive support by the WebKit team: WebKit/standards-positions#85