XHR API: Single event entries

[Elchi3]

Second attempt to merge XHR events to single entries.
See #13598 (comment) for context

Here I'm removing XMLHttpRequestEventTarget entirely and events are installed on XMLHttpRequest and XMLHttpRequestUpload. I'm assuming same compat data, but I think @foolip assumes they are different. Happy to adjust.

[Elchi3]

I'm assuming same compat data, but I think @foolip assumes they are different. Happy to adjust.

The consistency checker already found out that I was wrong. I adjusted to the versions of the main XMLHttpRequestUpload feature.

[foolip]

Here I'm removing XMLHttpRequestEventTarget entirely and events are installed on XMLHttpRequest and XMLHttpRequestUpload. I'm assuming same compat data, but I think @foolip assumes they are different. Happy to adjust.

Indeed, because the upload property was added later than XHR itself, so all of the XMLHttpRequestUpload data should be clamped to those version. However, I see they're currently inconsistent, which we need to sort out first I think. @queengooborg can you confirm the correct versions for api.XMLHttpRequest.upload? I assume that the Safari data in particular is wrong, this must be older than Safari 10...

[queengooborg]

It looks like the upload property has been supported in Safari since Safari 4, so our data is incorrect. I've opened #13874!

[ddbeck]

@Elchi3 This is blocked by merge conflicts, in case you hadn't seen so yet.