Re: [whatwg/dom] Event not dispatched by the user agent should not fallback to legacy (#404)

Here's a few links to bugs and code that convinced us to add the legacy invoke hacks in [Bug 1236979](https://bugzilla.mozilla.org/show_bug.cgi?id=1236979):

The biggest issue we know about is a map lib prioritized `webkitTransitionEnd` over `transitionend` when doing feature detection for prefixes, because some broken version of the Android browser had unprefixed relevant CSS properties, but did not unprefix events (i.e., would send prefixed events on an unprefixed transition or animation)

https://github.com/Leaflet/Leaflet/commit/64ca0af124b7145e8c2a746e2fb087b2298a4b72

```
+// webkitTransition comes first because some browser versions that drop vendor prefix don't do
+// the same for the transitionend event, in particular the Android 4.1 stock browser
```

Bing maps API usage breaking weather.com and strava, any other map using leaflet.js
https://bugzilla.mozilla.org/show_bug.cgi?id=1239342#c5
https://bugzilla.mozilla.org/show_bug.cgi?id=1236930

Possibly some versions of Modernizr (unconfirmed) had the same problem.
Also, bootstrap: https://github.com/twbs/bootstrap/blob/master/js/transition.js#L19

So, that said, for the known bustage, restricting invoking legacy events to trusted events *should* work. Of course, that doesn't guarantee anything for sites we don't know about that might be dispatching synthetic events. 😟 

But we could totally stick this in Nightly and see what bustage reports we get, and do some targeted testing.

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