Intermittent failure in /_mozilla/mozilla/mozbrowser/iframe_visibility.html

[simartin]

Found during a test run for #12565

  ▶ Unexpected subtest result in /_mozilla/mozilla/mozbrowser/iframe_visibility.html:
  │ FAIL [expected PASS] Requesting animation frame composites only when frame is visible
  │   → assert_true: expected true got false
  │ 
  │ iframe.onload</<@http://web-platform.test:8000/_mozilla/mozilla/mozbrowser/iframe_visibility.html:58:13
  │ Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:1402:20
  └ Test.prototype.step_func/<@http://web-platform.test:8000/resources/testharness.js:1426:20

[jdm]

Duplicate of #11825.