Intent to unship: HTTP Refresh header support when the content is handled as a download

[Matt Woodrow]

Hi,

In <https://bugzilla.mozilla.org/show_bug.cgi?id=1589270>Bug 1589270 I
intend to remove support for handling the HTTP Refresh header while
handling the content as a download.

This has been broken in our code with e10s enabled, since the initial
e10s conversion.

Neither Chrome nor Safari support this, and the only specified handling
of the Refresh header is part of initializing the document object [1],
which isn't run for downloads.

- Matt

[1]
https://html.spec.whatwg.org/multipage/browsing-the-web.html#initialise-the-document-object
<https://bugzilla.mozilla.org/show_bug.cgi?id=1589270>