Re: [whatwg/dom] Standardize certain Microsoft event extensions (#334)

A couple findings from running some `returnValue` tests:

* In Chrome and Safari Tech Preview, which implement both passive event listeners and `returnValue`, setting `{passive: true}` still allows the event to be canceled using `e.returnValue = false`. (Though I only tested a custom event, not a scroll event.)
  * This seems like an oversight which ought to be addressed as part of the spec.
* In Chrome, Safari, and Edge, setting `e.returnValue = false` appears to set `e.defaultPrevented = true` even when `e.cancelable = false`.
  * Which is to say that [this assertion fails](https://github.com/w3c/web-platform-tests/blob/778f2b4cd15ee747b6b38cef856f2f5c53cd3cd4/dom/events/Event-defaultPrevented.html#L22) when the test is switched from `preventDefault` to `returnValue`.
  * This would be contrary to the "nice" route of speccing `returnValue` as a simple alias of `preventDefault`, and is a wacky behavior in general. Hopefully no code actually relies on this...

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