Thread the status through navigation redirects

This is necessary because status codes affect whether the redirect is
done with the same HTTP method or a different one.

This is part of #21886, but there's also a flaw in how iframes are handled
that is causing the redirect to take over the entire window, so this
commit doesn't entirely fix slither.io.

components/constellation/network_listener.rs

@@ -113,6 +113,9 @@ impl NetworkListener {
                             location_url: metadata.location_url.clone(),
                             headers: headers.clone().into_inner(),
                             referrer: metadata.referrer.clone(),
+                            status_code: metadata.status.as_ref()
+                                .map(|&(code, _)| code)
+                                .unwrap_or(200),
                         });
 
                         // XXXManishearth we don't have the cancel_chan anymore and