Set timestamp to push messaging default notification.

This CL sets current timestamp to the default notification of push
messaging.
Previously, invalid time was shown when new style notification
chrome://flags/#enable-message-center-new-style-notification
is Enabled (which is ToT default).

TEST=manual
BUG=768695

Change-Id: Id253c990d39a8f8df71472c47137b8252a9186d4
Reviewed-on: https://chromium-review.googlesource.com/683914
Commit-Queue: Tetsui Ohkubo <tetsui@chromium.org>
Commit-Queue: Peter Beverloo <peter@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504296}

chrome/browser/push_messaging/push_messaging_notification_manager.cc

@@ -78,6 +78,7 @@ NotificationDatabaseData CreateDatabaseData(
       l10n_util::GetStringUTF16(IDS_PUSH_MESSAGING_GENERIC_NOTIFICATION_BODY);
   notification_data.tag = kPushMessagingForcedNotificationTag;
   notification_data.icon = GURL();
+  notification_data.timestamp = base::Time::Now();
   notification_data.silent = true;
 
   NotificationDatabaseData database_data;