Intermittent pass in /css/cssom-view/matchMedia.xht

[jdm]

  ▶ OK [expected TIMEOUT] /css/cssom-view/matchMedia.xht

  ▶ Unexpected subtest result in /css/cssom-view/matchMedia.xht:
  └ PASS [expected NOTRUN] Listeners are called in the order which they have been added

  ▶ Unexpected subtest result in /css/cssom-view/matchMedia.xht:
  └ PASS [expected NOTRUN] Listener added twice is only called once.

  ▶ Unexpected subtest result in /css/cssom-view/matchMedia.xht:
  │ FAIL [expected NOTRUN] Resize iframe from 200x100 to 200x50, then to 100x50
  │   → assert_equals: Check that the MediaQueryList passed to the handler is the same that addListener was invoked on. expected object "[object MediaQueryList]" but got object "[object MediaQueryListEvent]"
  │ 
  │ window.onload/</<@http://web-platform.test:8000/css/cssom-view/matchMedia.xht:132:25
  │ Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:1490:20
  └ window.onload/<@http://web-platform.test:8000/css/cssom-view/matchMedia.xht:131:21

This is probably a result of the changes in #20615.

[KiChjang]

Hmm... This sort of hints to me that there is a race condition when trying to get the window size of the iframe.