Bug 1531838 - Reliably wait for click-to-play popup notification in private windows. r=johannh

Differential Revision: https://phabricator.services.mozilla.com/D26700

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

browser/base/content/test/plugins/browser_private_clicktoplay.js

@@ -110,7 +110,9 @@ add_task(async function test2c() {
   await createPrivateWindow(gHttpTestRoot + "plugin_test.html");
   await topicObserved;
 
-  let popupNotification = gPrivateWindow.PopupNotifications.getNotification("click-to-play-plugins", gPrivateBrowser);
+  let popupNotification = await TestUtils.waitForCondition(() => {
+    return gPrivateWindow.PopupNotifications.getNotification("click-to-play-plugins", gPrivateBrowser);
+  }, "Waiting for click-to-play-plugins notification in the private window");
   ok(popupNotification, "Test 2c, Should have a click-to-play notification");
 
   await ContentTask.spawn(gPrivateBrowser, null, function() {
@@ -172,7 +174,9 @@ add_task(async function test3c() {
   await createPrivateWindow(gHttpTestRoot + "plugin_test.html");
   await topicObserved;
 
-  let popupNotification = gPrivateWindow.PopupNotifications.getNotification("click-to-play-plugins", gPrivateBrowser);
+  let popupNotification = await TestUtils.waitForCondition(() => {
+    return gPrivateWindow.PopupNotifications.getNotification("click-to-play-plugins", gPrivateBrowser);
+  }, "Waiting for click-to-play-plugins notification in the private window");
   ok(popupNotification, "Test 3c, Should have a click-to-play notification");
 
   // Check the button status