Use scheduler.postTask or analogue to assign tasks to background priority

[patmmccann]

A variety of events in the project are known background type priority, and we could use fetch keepalive, sendBeacon, or scheduler.postTask to label them as such to each browser type. Examples include #12387 and #10062 is related.

Any sort of analytics call or onTimeout or onError event triggering network traffic seems like a candidate for background processing.

Alternatively, we could take control of the timing even more explicitly and add delays or tie the events to some milestone.

[patmmccann]

committee feedback is that it is important that we could override this somwhow, that bidders or publishers could up the priority of things that were default background