Function FetchLater 

pub(crate) fn FetchLater(
    cx: &mut JSContext,
    window: &Window,
    input: RequestInfo<DomTypeHolder>,
    init: RootedTraceableBox<DeferredRequestInit<DomTypeHolder>>,
) -> Fallible<DomRoot<FetchLaterResult>>

https://fetch.spec.whatwg.org/#dom-window-fetchlater