Bug 1179397 - Disallow FetchEvent.respondWith() when the dispatch flag is unset; r=jdm

dom/workers/ServiceWorkerEvents.cpp

@@ -412,7 +412,7 @@ RespondWithHandler::CancelRequest(nsresult aStatus)
 void
 FetchEvent::RespondWith(Promise& aArg, ErrorResult& aRv)
 {
-  if (mWaitToRespond) {
+  if (EventPhase() == nsIDOMEvent::NONE || mWaitToRespond) {
     aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
     return;
   }

testing/web-platform/mozilla/meta/service-workers/service-worker/fetch-event-async-respond-with.https.html.ini

@@ -1,5 +0,0 @@
-[fetch-event-async-respond-with.https.html]
-  type: testharness
-  [Calling respondWith asynchronously throws an exception]
-    expected: FAIL
-

testing/web-platform/mozilla/tests/service-workers/service-worker/fetch-event-async-respond-with.https.html

@@ -22,8 +22,8 @@ promise_test(function(t) {
             });
           var worker = frame.contentWindow.navigator.serviceWorker.controller;
 
-          frame.remove();
           worker.postMessage({port: channel.port2}, [channel.port2]);
+          frame.remove();
           return saw_message;
         })
       .then(function(message) {