Bug 1579623 - disable test_fullscreen-api.html on Mac_64_debug for frequent failures. r=intermittent-reviewers,ahal DONTBUILD

Differential Revision: https://phabricator.services.mozilla.com/D115310

dom/html/test/mochitest.ini

@@ -439,7 +439,9 @@ skip-if = toolkit == 'android'
 [test_formelements.html]
 [test_fullscreen-api.html]
 tags = fullscreen
-skip-if = toolkit == 'android'
+skip-if =
+  toolkit == 'android'
+  os == 'mac' && bits == 64 && debug && webrender #Bug 1579623
 support-files =
   file_fullscreen-api.html
   file_fullscreen-async.html