Implement the Notification `timestamp` property

chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationBuilderBase.java

 // 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.
 
 package org.chromium.chrome.browser.notifications;
 
 import android.app.Notification;
 import android.app.PendingIntent;
 import android.graphics.Bitmap;
 import android.support.v4.app.NotificationCompat.Action;
@@ skipping 29 matching lines @@
     protected CharSequence mOrigin;
     protected CharSequence mTickerText;
     protected Bitmap mLargeIcon;
     protected int mSmallIconId;
     protected PendingIntent mContentIntent;
     protected PendingIntent mDeleteIntent;
     protected List<Action> mActions = new ArrayList<>(MAX_ACTION_BUTTONS);
     protected Action mSettingsAction;
     protected int mDefaults = Notification.DEFAULT_ALL;
     protected long[] mVibratePattern;
+    protected long mTimestamp;
 
     /**
      * Combines all of the options that have been set and returns a new Notification object.
      */
     public abstract Notification build();
 
     /**
      * Sets the title text of the notification.
      */
     public NotificationBuilderBase setTitle(@Nullable CharSequence title) {
@@ skipping 97 matching lines @@
     }
 
     /**
      * Sets the vibration pattern to use.
      */
     public NotificationBuilderBase setVibrate(long[] pattern) {
         mVibratePattern = Arrays.copyOf(pattern, pattern.length);
         return this;
     }
 
+    /**
+     * Sets the timestamp at which the event of the notification took place.
+     */
+    public NotificationBuilderBase setTimestamp(long timestamp) {
+        mTimestamp = timestamp;
+        return this;
+    }
+
     @Nullable
     private static CharSequence limitLength(@Nullable CharSequence input) {
         if (input == null) {
             return input;
         }
         if (input.length() > MAX_CHARSEQUENCE_LENGTH) {
             return input.subSequence(0, MAX_CHARSEQUENCE_LENGTH);
         }
         return input;
     }
 }