Take notification API out of experimental.

chrome/test/data/extensions/api_test/notification/api/events/background.js

-// Copyright (c) 2012 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-const notification = chrome.experimental.notification;
-
-var idString = "foo";
-
-var testBasicEvents = function() {
-  var incidents = 0;
-
-  var onCreateCallback = function(id) {
-    chrome.test.assertTrue(id.length > 0);
-    chrome.test.assertEq(idString, id);
-    incidents++;
-  }
-
-  var onDisplayed = function(id) {
-    incidents++;
-    if (incidents == 2) {
-      chrome.test.assertEq(idString, id);
-      chrome.test.succeed();
-    }
-  }
-  notification.onDisplayed.addListener(onDisplayed);
-
-  var options = {
-    templateType: "basic",
-    iconUrl: "/icon.png",
-    title: "Attention!",
-    message: "Check out Cirque du Soleil"
-  };
-  notification.create(idString, options, onCreateCallback);
-};
-
-chrome.test.runTests([ testBasicEvents ]);