Bug 1704572 - Use openMenu(true) instead of openPopup() for opening the 'this frame' submenu in browser/base/content/test/contextMenu/browser_contextmenu_iframe.js. r=Gijs

openMenu(true) works on both native and non-native context menus. openPopup() does not work on native context menus.

Differential Revision: https://phabricator.services.mozilla.com/D111682