Creates the nsDocShellLoadState object, which is basically
nsDocShellLoadInfo plus a few extra fields to make it usable as a
single argument to nsDocShell::LoadURI (and eventually
nsDocShell::InternalLoad).

Subframe history handling is a huge logic block in
nsDocShell::LoadURI, which is only used on history loads. This patch
also extracts the logic out into its own function to make the body of
LoadURI clearer.