merge_pr_45773

Make cross-origin-ancestor-activeelement-after-child-lose-focus.sub.html more robust.

This makes two changes to try to make the test
cross-origin-ancestor-activeelement-after-child-lose-focus.sub.html more
robust:

• It checks the message data in case postMessage is used by something
  else in WPT (although I think this is unlikely), to make sure that the
  message received is the one intended (that is sent by the subframe's
  subframe).
• It makes the subframe's subframe wait until load before focusing its
  input, on the theory that it might be possible that this focusing is
  happening too early to be effective.

My hope is that these might fix the test being flaky in Chromium CI.

Bug: 332068449
Change-Id: Idf70ba87a8559223f9dff7900cadca9ea56126e4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5464001
Auto-Submit: David Baron dbaron@chromium.org
Commit-Queue: Di Zhang dizhangg@chromium.org
Reviewed-by: Di Zhang dizhangg@chromium.org
Cr-Commit-Position: refs/heads/main@{#1288995}