Bug 1548687 - Remove unused dangling argument. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D35203

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

gfx/layers/apz/test/mochitest/browser_test_group_fission.js

@@ -10,12 +10,12 @@ add_task(async function test_main() {
   // Each of these URLs will get opened in a new top-level browser window that
   // is fission-enabled.
   var test_urls = [
-    httpURL("helper_fission_basic.html", null),
+    httpURL("helper_fission_basic.html"),
     // add additional tests here
   ];
   if (isWebRender) {
     test_urls = test_urls.concat([
-      httpURL("helper_fission_transforms.html", null),
+      httpURL("helper_fission_transforms.html"),
       // add additional WebRender-specific tests here
     ]);
   }