Bug 1434599 - Enable test_workerupdatefoundevent.html. r=bkelly

dom/serviceworkers/test/mochitest.ini

@@ -341,7 +341,6 @@ tags = openwindow
 [test_workerUnregister.html]
 [test_workerUpdate.html]
 [test_workerupdatefoundevent.html]
-skip-if = !e10s # Bug 1433276
 [test_xslt.html]
 [test_async_waituntil.html]
 [test_worker_reference_gc_timeout.html]

dom/serviceworkers/test/worker_updatefoundevent.js

@@ -6,7 +6,9 @@
 registration.onupdatefound = function(e) {
   clients.matchAll().then(function(clients) {
     if (!clients.length) {
-      reject("No clients found");
+      // We don't control any clients when the first update event is fired
+      // because we haven't reached the 'activated' state.
+      return;
     }
 
     if (registration.scope.match(/updatefoundevent\.html$/)) {