Works fine for me in Ubuntu 14.04 with Unity as my Desktop environment and using the latest packaged version of Chromium.

It only comes up if you receive a notification from a website: http://imgur.com/lGOJrHb

You can then click 'Chromium - Notifications' to get what I believe to be the 'notification center': http://imgur.com/JyIVNvG

you can try it out if you open the chromium console (f12), and enter "setTimeout(() => new Notification("Insert text here"), 2000)" and alt-tab to something that isn't chromium.

Neat feature, but hard to expect users to figure it out on their own.