Bug 1492482 - Remove CPOW usage from browser_aboutnewtab_process_selection.js. r=Felipe

Depends on D6967

Differential Revision: https://phabricator.services.mozilla.com/D6968

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

dom/base/test/browser_aboutnewtab_process_selection.js

@@ -18,9 +18,10 @@ async function ensurePreloaded(gBrowser) {
   gBrowser._createPreloadBrowser();
   // We cannot use the regular BrowserTestUtils helper for waiting here, since that
   // would try to insert the preloaded browser, which would only break things.
-  await BrowserTestUtils.waitForCondition( () => {
-    let doc = gBrowser._preloadedBrowser.contentDocumentAsCPOW;
-    return doc && doc.readyState == "complete";
+  await ContentTask.spawn(gBrowser._preloadedBrowser, null, async () => {
+    await ContentTaskUtils.waitForCondition(() => {
+      return content.document && content.document.readyState == "complete";
+    });
   });
 }