Intermittent failure in /xhr/open-url-multi-window-5.htm

[jdm]

  ▶ Unexpected subtest result in /xhr/open-url-multi-window-5.htm:
  │ FAIL [expected PASS] XMLHttpRequest: open() resolving URLs (multi-Window; 5)
  │   → assert_throws: function "function() { client.open("GET", "...") }" did not throw
  │ 
  │ init/<@http://web-platform.test:8000/xhr/open-url-multi-window-5.htm:22:13
  │ Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:1587:20
  │ init@http://web-platform.test:8000/xhr/open-url-multi-window-5.htm:16:9
  └ onload/<@http://web-platform.test:8000/xhr/resources/init.htm:12:13