[Extensions] Update web API cloberring for platform apps

chrome/test/data/extensions/app_forbidden_apis/onbeforeunload/background.js

+// Copyright 2016 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.
+
+'use strict';
+
+var didRun = sessionStorage.didRun;
+
+function beforeUnload() {
+  chrome.test.fail();
+}
+
+try {
+  delete window.onbeforeunload;
+  window.onbeforeunload = beforeUnload;
+} catch (e) {}
+
+try {
+  window.addEventListener('beforeunload', beforeUnload);
+} catch (e) {}
+
+if (!didRun) {
+  sessionStorage.didRun = true;
+  location.reload();
+} else {
+  chrome.test.succeed();
+}