Implement the Notification `timestamp` property

third_party/WebKit/LayoutTests/http/tests/notifications/notification-properties.html

Index: third_party/WebKit/LayoutTests/http/tests/notifications/notification-properties.html
diff --git a/third_party/WebKit/LayoutTests/http/tests/notifications/notification-properties.html b/third_party/WebKit/LayoutTests/http/tests/notifications/notification-properties.html
index fbe714a3a125e00c9ae4f12fc4121ec45c77f417..5fb05b9fadefb2307cd5a8e24b37b2b506b55dcc 100644
--- a/third_party/WebKit/LayoutTests/http/tests/notifications/notification-properties.html
+++ b/third_party/WebKit/LayoutTests/http/tests/notifications/notification-properties.html
@@ -22,6 +22,7 @@
                 body: "Hallo, wereld!",
                 tag: "notification",
                 icon: "http://localhost/my_icon.png",
+                timestamp: 621046800000,
                 silent: true,
                 requireInteraction: true,
                 data: "my data",
@@ -36,6 +37,7 @@
             assert_equals(notification.body, options.body);
             assert_equals(notification.tag, options.tag);
             assert_equals(notification.icon, options.icon);
+            assert_equals(notification.timestamp, options.timestamp);
             assert_true(notification.silent);
             assert_true(notification.requireInteraction);
             assert_equals(notification.data, options.data);
@@ -55,6 +57,9 @@
             assert_equals(emptyNotification.data, null);
             assert_array_equals(emptyNotification.actions, []);
 
+            var timeDifference = Math.abs(Date.now() - emptyNotification.timestamp);
+            assert_true(timeDifference < 16);  // 16 is not significant, just to reduce flakiness.
+
             var equalNotification = new Notification("My Notification", {
                 vibrate: [50, 10, 50, 10, 50],
                 data: { hello: "World!" }