[whatwg/dom] cancelBubble legacy property (#211)

[`UIEvent.cancelBubble`](https://developer.mozilla.org/en-US/docs/Web/API/UIEvent/cancelBubble) works in all modern browsers.
Testcase: http://jsbin.com/jufexu/edit?html,css,js,output

`Event.cancelBubble` works in all modern browsers (Chrome, Safari, Edge, IE11) except Firefox.
Testcase: http://jsbin.com/wirelo/edit?html,js,output

To accurately reflect reality, I propose speccing at least one of these properties.

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