Bug 1557712: Fix intermittent test failure for test_frameancestors_userpass.html. r=jkt

Differential Revision: https://phabricator.services.mozilla.com/D50907

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

dom/security/test/csp/test_frameancestors_userpass.html

@@ -95,7 +95,10 @@ var frameLoaded = function(testname, uri) {
 var frameBlocked = function(uri, policy) {
 
   //Check if @ symbol is there in URI or in csp policy.
-  if (policy.includes('@') || uri.includes('@')) {
+  // Bug 1557712: Intermittent failure -> not sure why the 'uri' might ever
+  // be non existing at this point, however if there is no uri, there can
+  // also be no userpass!
+  if (policy.includes('@') || (uri && uri.includes('@'))) {
     ok(false, ' a CSP policy blocked frame from being loaded. But contains' +
       ' userpass. Policy is: ' + policy + ';URI is: ' + uri );
   } else {