Fix build

Looks like I mishandled a merge conflict?
If a tab is created only for a download, close it

If a tab that has just been created ends up being just a download, then
close it down. If it's the only one on the window, then make it show the
overview instead.

https://bugzilla.gnome.org/show_bug.cgi?id=607233
Use a flag to decide if a tab is download-only

There are some cases in which a view may not be able to go back and still be not
just for a download: if the view has been opened by middle clicking somewhere or
by having an URL provided on startup, then the navigation from the overview to
the first page won't have happened. By tracking whether the page has ever
committed a load we can be sure it has been opened just for the download.

https://bugzilla.gnome.org/show_bug.cgi?id=720609