Bug 853747 - Set dom.use_xbl_scopes_for_remote_xul for b2g reftests, r=bholley

layout/tools/reftest/b2g_start_script.js

@@ -9,6 +9,7 @@ function setDefaultPrefs() {
     var prefs = Components.classes["@mozilla.org/preferences-service;1"].
                 getService(Components.interfaces.nsIPrefService);
     var branch = prefs.getDefaultBranch("");
+    branch.setBoolPref("dom.use_xbl_scopes_for_remote_xul", true);
     branch.setBoolPref("gfx.color_management.force_srgb", true);
     branch.setBoolPref("browser.dom.window.dump.enabled", true);
     branch.setIntPref("ui.caretBlinkTime", -1);