LibWeb: Ensure that replaced documents keep the Window of their sibling

The note in the spec says that we're supposed to make sure this new
document has the same Window as the old about:blank document, but we
forgot to actually assign to the Window pointer.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -153,6 +153,8 @@ JS::NonnullGCPtr<Document> Document::create_and_initialize(Type type, String con
         && navigation_params.history_handling == HTML::HistoryHandlingBehavior::Replace
         && (browsing_context->active_document() && browsing_context->active_document()->origin().is_same_origin(navigation_params.origin))) {
         // Do nothing.
+        // NOTE: This means that both the initial about:blank Document, and the new Document that is about to be created, will share the same Window object.
+        window = browsing_context->active_window();
     }
 
     // 6. Otherwise: