Intermittent timeout /html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events-iframe.html

[ferjm]

{
    "status": "TIMEOUT", 
    "group": "default", 
    "message": "Test timed out", 
    "stack": null, 
    "subtest": "delayed handling: promise is created in iframe and being rejected elsewhere", 
    "test": "/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events-iframe.html", 
    "line": 215106, 
    "action": "test_result", 
    "expected": "PASS"
}
{
    "status": "TIMEOUT", 
    "group": "default", 
    "message": null, 
    "stack": null, 
    "subtest": null, 
    "test": "/html/webappapis/scripting/processing-model-2/unhandled-promise-rejections/promise-rejection-events-iframe.html", 
    "line": 215107, 
    "action": "test_result", 
    "expected": "OK"
}

[pshaughn]

While #22295 and #22207 had races in the test logic, I think this one is performance. Running it on my slow VM, the harness timed out but then a while later the tests actually passed in the browser window.