Bug 1578041 - Re-mark test_ext_cookies.html as failing with Fission. r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D44489

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

toolkit/components/extensions/test/mochitest/mochitest-common.ini

@@ -85,6 +85,7 @@ skip-if = (!debug && android_version == '18') #Bug 1523193
 skip-if = os == 'android' # Android does not support multiple windows.
 [test_ext_contentscript_permission.html]
 [test_ext_cookies.html]
+fail-if = fission
 [test_ext_cookies_containers.html]
 [test_ext_cookies_expiry.html]
 [test_ext_cookies_first_party.html]