Re: [whatwg/dom] document.createEvent() supports lots of events that most UAs do not support initializers for (#362)

Usage of HashChangeEvent is [negligible](https://telemetry.mozilla.org/new-pipeline/evo.html#!aggregates=submissions&cumulative=0&end_date=2016-08-28&keys=!__none__!__none__&max_channel_version=nightly%252F51&measure=CREATE_EVENT_HASHCHANGEEVENT&min_channel_version=nightly%252F48&processType=*&product=Firefox&sanitize=0&sort_keys=submissions&start_date=2016-08-22&trim=0&use_submission_date=0) in Firefox, so I don't mind removing that too.  DragEvent is [low but not as low](https://telemetry.mozilla.org/new-pipeline/evo.html#!aggregates=submissions&cumulative=0&end_date=2016-08-28&keys=!__none__!__none__&max_channel_version=nightly%252F51&measure=CREATE_EVENT_DRAGEVENT&min_channel_version=nightly%252F48&processType=*&product=Firefox&sanitize=0&sort_keys=submissions&start_date=2016-08-22&trim=0&use_submission_date=0) and I don't think we want to get rid of it unless there's good reason.  I'm 

We have super-high usage for (BeforeUnloadEvent)[https://telemetry.mozilla.org/new-pipeline/evo.html#!aggregates=submissions&cumulative=0&end_date=2016-08-28&keys=!__none__!__none__&max_channel_version=nightly%252F51&measure=CREATE_EVENT_BEFOREUNLOADEVENT&min_channel_version=nightly%252F48&processType=*&product=Firefox&sanitize=0&sort_keys=submissions&start_date=2016-08-22&trim=0&use_submission_date=0], but that's probably because we use it internally.  I'd have to remove the internal user in order to have useful telemetry.  But we'd also have to add a constructor here, no?  Or do we want the event to be fired by the browser but authors can't create it?

@annevk So it looks like the following are all either unsupported in Firefox/Chrome/Safari already, or we're willing to remove support:

* AnimationEvent
* CloseEvent
* ErrorEvent
* IDBVersionChangeEvent
* PageTransitionEvent
* PopStateEvent
* ProgressEvent
* TrackEvent
* TransitionEvent (foolip still doesn't seem 100% sure if Chrome can drop it)
* WebGLContextEvent
* WheelEvent (supported by Safari, need buy-in from them)

Edge supports all of these events, I'm guessing just because they're following the spec and not because sites depend on them.

So I think the right course of action now is to ask Safari if they're willing to drop support for WheelEvent, and get confirmation from Chrome that they can drop TransitionEvent, and then drop all of these from the spec.

We also need to figure out what to do about BeforeUnloadEvent.  It's fired by the browser, and has no constructor, so do we want to keep createEvent support?  Or add a constructor?  Or not let authors create it although the browser fires it?  (Is the last what Edge/Safari do now?)

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