Support persistent notifications in the PlatformNotificationServiceImpl.

chrome/test/data/notifications/platform_notification_service.js

+// Service Worker to be used with the platform_notification_service.html page.
+var messagePort = null;
+
+addEventListener('message', function (event) {
+  messagePort = event.data;
+  messagePort.postMessage('ready');
+});
+
+// The notificationclick event will be invoked when a persistent notification
+// has been clicked on. When this happens, the title determines whether this
+// Service Worker has to act upon this.
+addEventListener('notificationclick', function (event) {
+  if (event.notification.title == 'action_close')
+    event.notification.close();
+
+  messagePort.postMessage(event.notification.title);
+});