Bug 1578039 - Re-disable test_postMessage_onOther.html with Fission. r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D44490

--HG--
extra : moz-landing-system : lando

dom/tests/mochitest/whatwg/mochitest.ini

@@ -33,6 +33,7 @@ skip-if = fission # Timeouts
 [test_postMessage_joined.html]
 skip-if = fission # Timeouts
 [test_postMessage_onOther.html]
+skip-if = fission #Bug 1571273
 [test_postMessage_origin.xhtml]
 skip-if = fission # Timeouts
 [test_postMessage_override.html]