Fix dockMenu not being referenced in JavaScript (#12062)

* Fix dockMenu not being referenced in JavaScript

* spec: Test garbage collecting dock menu

lib/browser/api/app.js

@@ -10,6 +10,8 @@ const electron = require('electron')
 const {deprecate, Menu} = electron
 const {EventEmitter} = require('events')
 
+let dockMenu = null
+
 // App is an EventEmitter.
 Object.setPrototypeOf(App.prototype, EventEmitter.prototype)
 EventEmitter.call(app)
@@ -49,7 +51,13 @@ if (process.platform === 'darwin') {
     hide: bindings.dockHide,
     show: bindings.dockShow,
     isVisible: bindings.dockIsVisible,
-    setMenu: bindings.dockSetMenu,
+    setMenu (menu) {
+      dockMenu = menu
+      bindings.dockSetMenu(menu)
+    },
+    getMenu () {
+      return dockMenu
+    },
     setIcon: bindings.dockSetIcon
   }
 }