Remove config.mainWindowID tests from common/*.

common/content/configbase.js

@@ -91,6 +91,11 @@ const configbase = { //{{{
      */
     ignoreKeys: {}, // XXX: be aware you can't put useful values in here, as "modes.NORMAL" etc. are not defined at this time
 
+    /**
+     * @property {string} The ID of the application's main XUL window.
+     */
+    mainWindowID: document.documentElement.id,
+
     /**
      * @property {[[]]} An array of application specific mode specifications.
      *     The values of each mode are passed to modes.addMode during