QWebEngineDownloadItem::downloadProgress has 0 as bytesTotal if unknown

When downloading a file with an unknown size like http://httpbin.org/stream-bytes/1024 with QtWebEngine, downloadProgress gets emitted with 0 for bytesTotal.

This is kind of unexpected and different from QNetworkReply::downloadProgress where -1 is used for that purpose.

It's probably too late to change that behaviour now, but I think it should at least be documented.