The relevant information was already being set on the final channel when
not performing a process switch [1], but it was not copied for
cross-process redirects, despite being sent to the child process. This
wasn't a problem previously, as we would handle downloads in the parent
process, but with the download panel improvements, it is an issue.
This issue led to the wrong filename being selected by default when
clicking on the save icon in a downloaded PDF from a blob URL, as the
information was lost before being passed to the pdf.js stream filter.
[1]: https://searchfox.org/mozilla-central/rev/87ecd21d3ca517f8d90e49b32bf042a754ed8f18/netwerk/ipc/DocumentChannelChild.cpp#308-315
Differential Revision: https://phabricator.services.mozilla.com/D145255
22da0176b0