Function window_for_replacement 

fn window_for_replacement(
    script_window_proxies: &ScriptWindowProxies,
    id: BrowsingContextId,
    origin: &MutableOrigin,
) -> Option<DomRoot<Window>>

Steps 1, 5, and 6 of https://html.spec.whatwg.org/multipage/#initialise-the-document-object