Add "app.whenReady()" (#12652)

* Make "chai-as-promised" avaialble in tests

* Add "app.whenReady()"

Closes #9561.

lib/browser/api/app.js

@@ -11,12 +11,30 @@ const {deprecate, Menu} = electron
 const {EventEmitter} = require('events')
 
 let dockMenu = null
+let readyPromise = null
 
 // App is an EventEmitter.
 Object.setPrototypeOf(App.prototype, EventEmitter.prototype)
 EventEmitter.call(app)
 
 Object.assign(app, {
+  whenReady () {
+    if (readyPromise !== null) {
+      return readyPromise
+    }
+
+    if (app.isReady()) {
+      readyPromise = Promise.resolve()
+    } else {
+      readyPromise = new Promise(resolve => {
+        // XXX(alexeykuzmin): Explicitly ignore any data
+        // passed to the event handler to avoid memory leaks.
+        app.once('ready', () => resolve())
+      })
+    }
+
+    return readyPromise
+  },
   setApplicationMenu (menu) {
     return Menu.setApplicationMenu(menu)
   },