[Fetch-Based API] sendAfterBeingBackgroundedTimeout and deferSend

[mingyc]

Context: #70 and #71

class DeferSend {
  constructor(sendAfterBeingBackgroundedTimeout)
}

Current proposal is to make deferSend a class, and sendAfterBeingBackgroundedTimeout its optional field.

1. Should this be a standalone option in [RequestInit]? But it is not relevant to other existing fetch options.
2. Should the timeout support sending after hidden or pagehide (bfcached)? (Previous discussion in Should pageHideTimeout support additional events #13).
3. Need user input for how desirable for this option.
4. Need better naming suggestion.

[mingyc]

@yoavweiss @clelland @philipwalton @annevk Could you please take a look?

[philipwalton]

(Adding @tunetheweb to take a look while I'm on leave.)

[yoavweiss]

^^ @noamr

[yoavweiss]

@mingyc - I think this is quite different from the Controller-based design we discussed. Let's sync about this.

[mingyc]

The SendSignal controller is tracked in #75

[mingyc]

• sendAfterBeingBackgroundedTimeout is now activateAfter in fetchLater() API. See Deferred fetching whatwg/fetch#1647 (comment)
• deferSend is now the default behavior of fetchLater() API. See Deferred fetching https://whatpr.org/fetch/1647.html#deferred-fetching