[whatwg] [Notifications] Constructor should not have side effects

[Elliott Sprehn]

The Notification constructor should not have side effects. This is
generally considered bad design, and the rest of the platform doesn't have
this either.

Specifically new Notification() should not show the notification since it
prevents reuse of the notification after calling close(), and is surprising
behavior.

(ex. new XMLHttpRequest doesn't fire the request, new HTMLDivElement()
shouldn't append it to the document, ...)

- E