Implement the Notification `timestamp` property

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

Index: chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationBuilderBase.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationBuilderBase.java b/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationBuilderBase.java
index 8afc53a5a1334514ae2265f166bda778d6e2c4d7..08ef5fb47bd1fc05f5ea60dc671ced168090cdcb 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationBuilderBase.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/notifications/NotificationBuilderBase.java
@@ -47,6 +47,7 @@ public abstract class NotificationBuilderBase {
     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.
@@ -164,6 +165,14 @@ public abstract class NotificationBuilderBase {
         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) {