[Extensions] Make port creation synchronous in unload handlers

chrome/test/data/extensions/api_test/messaging/on_unload/test.js

Index: chrome/test/data/extensions/api_test/messaging/on_unload/test.js
diff --git a/chrome/test/data/extensions/api_test/messaging/on_unload/test.js b/chrome/test/data/extensions/api_test/messaging/on_unload/test.js
new file mode 100644
index 0000000000000000000000000000000000000000..cbef07f1ddec2992f1de4b23642ac5b99e0d334a
--- /dev/null
+++ b/chrome/test/data/extensions/api_test/messaging/on_unload/test.js
@@ -0,0 +1,10 @@
+// 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.
+
+window.messageCount = 0;
+
+chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
+  ++window.messageCount;
+  window.message = request;
+});