[whatwg/xhr] Clarify if `ProgressEvent.loaded` should indicate the size of compress, or uncompressed, data (Issue #388)

### What is the issue with the XMLHttpRequest Standard?

When a [`ProgressEvent`](https://xhr.spec.whatwg.org/#interface-progressevent) fires, `ProgressEvent.loaded` contains the size of the data already transmitted.

If the data is compressed and its total size is unknown, then, **depending on the browser, `ProgressEvent.loaded` can contain either the size of the compressed data, or the size of the decompressed data**.

It would be great if the spec could clarify which of these two values should be the one provided by `ProgressEvent.loaded`.

### Additional context

With a response containing compressed data and unknown total size (the `Content-Length` header is not set), `ProgressEvent.lengthComputable` is `false` and `ProgressEvent.total` is not usable, preventing us from displaying a progress indicator.

In this case, a [common technique](https://stackoverflow.com/questions/15097712/how-can-i-use-deflated-gzipped-content-with-an-xhr-onprogress-function/32799706#32799706) is to set a custom response header `X-Uncompressed-Content-Length` containing the size of the uncompressed data being transmitted.

We can then, on the client side, use the value of this header as a substitue of `ProgressEvent.total`, and display a progress indicator with the value of `ProgressEvent.loaded` divided by the value of this header.

However, depending on the browser, `ProgressEvent.loaded` can be either the size of the compressed data already transmitted (the case in Firefox), or, the size of the decompressed data already transmitted (the case in Chrome).

For example:
- In Chrome, downloading a content of 1.3MB (1,345,276 bytes), while logging the value of `loaded`, shows a total of 1345276 at the end.
- In Firefox, downloading the same content of 1,345,276 bytes, while logging the value of `loaded`, shows a total of 1017454 at the end.


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