Intermittent FAIL on /_mozilla/mozilla/xmlhttprequest_url.html: "Performance entries should contain the URL where the XMLHttpRequest originated"

[Manishearth]

  ▶ Unexpected subtest result in /_mozilla/mozilla/xmlhttprequest_url.html:
  │ FAIL [expected PASS] Performance entries should contain the URL where the XMLHttpRequest originated
  │   → assert_equals: expected "http://web-platform.test:8000/_mozilla/mozilla/xmlhttprequest_url.html" but got "http://web-platform.test:8000/resources/testharness.js"
  │ 
  │ request.onload<@http://web-platform.test:8000/_mozilla/mozilla/xmlhttprequest_url.html:23:5
  │ Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:1594:25
  └ Test.prototype.step_func_done/<@http://web-platform.test:8000/resources/testharness.js:1634:32

#23641 (comment)