Bug 1439875: Update browser_windowopen_reflows.js to not wait for a resize that no longer exists. r=florian

MozReview-Commit-ID: Jln9ejZh2b6

--HG--
extra : rebase_source : 7e3eafa77fae784433dffc80ad20dd02a55f1141
extra : source : 8793a244abce5dab717769cbec698a6ece9c28c5
extra : histedit_source : aad1c97c5fe726c1a4ddd35b43aec7224c7e601c

browser/base/content/test/performance/browser_windowopen_reflows.js

@@ -62,12 +62,8 @@ add_task(async function() {
   let win = OpenBrowserWindow();
 
   await withReflowObserver(async function() {
-    let resizeEvent = BrowserTestUtils.waitForEvent(win, "resize");
-    let delayedStartup =
-      TestUtils.topicObserved("browser-delayed-startup-finished",
-                              subject => subject == win);
-    await resizeEvent;
-    await delayedStartup;
+    await TestUtils.topicObserved("browser-delayed-startup-finished",
+                                  subject => subject == win);
   }, EXPECTED_REFLOWS, win);
 
   await BrowserTestUtils.closeWindow(win);