Implement the Notification `timestamp` property

chrome/android/javatests/src/org/chromium/chrome/browser/notifications/NotificationUIManagerTest.java

 // Copyright 2015 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.
 
 package org.chromium.chrome.browser.notifications;
 
 import android.annotation.SuppressLint;
 import android.app.Notification;
 import android.content.Context;
 import android.content.res.Resources;
@@ skipping 42 matching lines @@
         assertEquals("Hello", notification.extras.getString(Notification.EXTRA_TEXT));
         assertEquals(UrlUtilities.formatUrlForSecurityDisplay(getOrigin(), false /* showScheme */),
                 notification.extras.getString(Notification.EXTRA_SUB_TEXT));
 
         // Verify that the ticker text contains the notification's title and body.
         String tickerText = notification.tickerText.toString();
 
         assertTrue(tickerText.contains("MyNotification"));
         assertTrue(tickerText.contains("Hello"));
 
+        // Verify that the notification's timestamp is set in the past 60 seconds. This number has
+        // no significance, but needs to be high enough to not cause flakiness as it's set by the
+        // renderer process on notification creation.
+        assertTrue(Math.abs(System.currentTimeMillis() - notification.when) < 60 * 1000);
+
         // Validate the appearance style of the notification. The EXTRA_TEMPLATE was introduced
         // in Android Lollipop, we cannot verify this in earlier versions.
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP
                 && !NotificationUIManager.useCustomLayouts()) {
             assertEquals("android.app.Notification$BigTextStyle",
                     notification.extras.getString(Notification.EXTRA_TEMPLATE));
         }
 
         assertNotNull(notification.largeIcon);
 
@@ skipping 225 matching lines @@
         assertEquals(1, notifications.size());
         assertEquals("Two",
                 notifications.get(0).notification.extras.getString(Notification.EXTRA_TITLE));
 
         // Close the last notification and verify that none remain.
         notifications.get(0).notification.contentIntent.send();
         waitForNotificationManagerMutation();
         assertTrue(getNotificationEntries().isEmpty());
     }
 }