Add spec for devtools extension

spec/api-browser-window-spec.js

@@ -824,6 +824,30 @@ describe('browser-window module', function () {
   })
 
   describe('dev tool extensions', function () {
+    describe('BrowserWindow.addDevToolsExtension', function () {
+      it('creates the extension', function (done) {
+        var extensionPath = path.join(__dirname, 'fixtures', 'devtools-extensions', 'foo')
+        BrowserWindow.addDevToolsExtension(extensionPath)
+
+        w.webContents.on('devtools-opened', () => {
+          var inputEventIntervalId = setInterval(function () {
+            if (w && w.devToolsWebContents) {
+              w.devToolsWebContents.sendInputEvent({type: 'keyDown', keyCode:'[', modifiers: ['meta']})
+            } else {
+              clearInterval(inputEventIntervalId)
+            }
+          }, 250)
+        })
+        w.loadURL('about:blank')
+        w.webContents.openDevTools({mode: 'bottom'})
+
+        ipcMain.once('answer', function (event, message) {
+          assert.equal(message, 'extension loaded')
+          done()
+        })
+      })
+    })
+
     it('serializes the registered extensions on quit', function () {
       var extensionName = 'foo'
       var extensionPath = path.join(__dirname, 'fixtures', 'devtools-extensions', extensionName)