When a microtask is executed, we need to use an appropriate,
non-detached Context for its execution. Currently with
PromiseResolveThenableJobs [1], the Context used is always drawn from
the realm of the Promise constructor being used. This may cause
non-intuitive behavior, such as in the following case:

  const DeadPromise = iframe.contentWindow.Promise;
  const p = DeadPromise.resolve({
    then() {
      return { success: true };
    }
  });
  p.then(result => { console.log(result); });

  // Some time later, but synchronously...
  iframe.src = "http://example.com"; // navigate away.
  // DeadPromise's Context is detached state now.
  // p never gets resolved, and its reaction handler never gets called.

To fix this behavior, when PromiseResolveThenableJob is being queued up,
the `then` method of the thenable should be used to determine the
context of the resultant microtask. Doing so aligns with Firefox, and
also with the latest HTML spec [2][3].

This change is analogous to CL 1465902, which uses the realm of the
reaction handlers to determine the Context PromiseReactionJobs run in.

[1]: https://tc39.es/ecma262/#sec-promiseresolvethenablejob
[2]: https://html.spec.whatwg.org/C/#enqueuejob(queuename,-job,-arguments)
[3]: https://github.com/whatwg/html/pull/5212

Bug: v8:10200
Change-Id: I2312788eeea0f9e870c13cf3cb5730a87d15609e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2071624
Commit-Queue: Timothy Gu <timothygu@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66507}