Bug 1930220 - Assert nothing changes in the toolbar upon browser restarts r=mconley,webdriver-reviewers,whimboo

To help catch bugs such as Bug 1919721, assert the toolbar is unchanged and widgets are in the correct order on browser restart.

Differential Revision: https://phabricator.services.mozilla.com/D233749

browser/components/customizableui/CustomizableUI.sys.mjs

@@ -5010,6 +5010,17 @@ export var CustomizableUI = {
     // We need to clone this, as we don't want to let consumers muck with placements
     return [...gPlacements.get(aArea)];
   },
+  /**
+   * Get an array of all the widget IDs in the default placements for an area.
+   * Modifying the array will not affect CustomizableUI.
+   *
+   * @param aArea the ID of the area whose default placements you want to obtain.
+   * @return an array containing the widget IDs that are in the default placements for that area.
+   *
+   */
+  getDefaultPlacementsForArea(aArea) {
+    return [...gAreas.get(aArea).get("defaultPlacements")];
+  },
   /**
    * Get an array of widget wrappers for all the widgets in an area. This is
    * the same as calling getWidgetIdsInArea and .map() ing the result through