Bug 1381769 - fix xpcshell failure when data: URI is unique origin. r=smaug

ExtensionXPCShellUtils.jsm uses a XUL window with data: URI, we replace
it with a XUL file to fix the xpcshell-test failures.

toolkit/components/extensions/ExtensionXPCShellUtils.jsm

@@ -61,11 +61,6 @@ function frameScript() {
 
 const FRAME_SCRIPT = `data:text/javascript,(${encodeURI(frameScript)}).call(this)`;
 
-
-const XUL_URL = "data:application/vnd.mozilla.xul+xml;charset=utf-8," + encodeURI(
-  `<?xml version="1.0"?>
-  <window id="documentElement"/>`);
-
 let kungFuDeathGrip = new Set();
 function promiseBrowserLoaded(browser, url) {
   return new Promise(resolve => {
@@ -110,7 +105,7 @@ class ContentPage {
 
     chromeShell.createAboutBlankContentViewer(system);
     chromeShell.useGlobalHistory = false;
-    chromeShell.loadURI(XUL_URL, 0, null, null, null);
+    chromeShell.loadURI("chrome://extensions/content/dummy.xul", 0, null, null, null);
 
     await promiseObserved("chrome-document-global-created",
                           win => win.document == chromeShell.document);