Bug 1375710 - Remove CPOWs from browser_domFullscreen_fullscreenMode.js. r=Gijs

MozReview-Commit-ID: FCoCrJ8Y9jn

--HG--
extra : rebase_source : c7f6ca0264188116f86a6aa63fe0479ae992cf66

browser/base/content/test/general/browser_domFullscreen_fullscreenMode.js

@@ -163,18 +163,16 @@ add_task(async function() {
     ["full-screen-api.transition-duration.enter", "0 0"],
     ["full-screen-api.transition-duration.leave", "0 0"]);
 
-  let tab = BrowserTestUtils.addTab(gBrowser, kPage);
-  let browser = tab.linkedBrowser;
-  gBrowser.selectedTab = tab;
-  await waitForDocLoadComplete();
-
-  registerCleanupFunction(() => {
-    if (browser.contentWindow.fullScreen) {
-      BrowserFullScreen();
+  registerCleanupFunction(async function() {
+    if (window.fullScreen) {
+      executeSoon(() => BrowserFullScreen());
+      await waitForFullscreenChanges(FS_CHANGE_SIZE);
     }
-    gBrowser.removeTab(tab);
   });
 
+  let tab = await BrowserTestUtils.openNewForegroundTab({ gBrowser, url: kPage });
+  let browser = tab.linkedBrowser;
+
   gMessageManager = browser.messageManager;
   gMessageManager.loadFrameScript(
     "data:,(" + frameScript.toString() + ")();", false);
@@ -237,4 +235,6 @@ add_task(async function() {
       await waitForFullscreenChanges(FS_CHANGE_SIZE);
     }
   }
+
+  await BrowserTestUtils.removeTab(tab);
 });