Starting a load from a link click, to a link with the 'download' attribute
doesn't cancel any existing loads (since it's known to be just a download, not a
navigation, so doesn't need to replace the existing one). But then we have two
DocumentChannels alive at the same time, on the same loadgroup, the loadgroup
copies flags from the old one to the download one, since they're different, and
triggers the assert about setting LoadFlags.
The download one shouldn't really be added to the loadgroup; the progress of the
download should NOT block the 'load' event of the existing navigation.
Differential Revision: https://phabricator.services.mozilla.com/D85964
Dan Glastonbury