LibWeb: Call did_finish_load when page readiness changes to complete

Before, this function were called from FrameLoader and now we have to
move it to another place so it is still called after migrating to
navigables.

Userland/Libraries/LibWeb/DOM/Document.cpp

@@ -1893,6 +1893,12 @@ void Document::update_readiness(HTML::DocumentReadyState readiness_value)
 
     // 4. Fire an event named readystatechange at document.
     dispatch_event(Event::create(realm(), HTML::EventNames::readystatechange));
+
+    if (readiness_value == HTML::DocumentReadyState::Complete && is_active() && navigable()->is_traversable()) {
+        if (auto* page = navigable()->traversable_navigable()->page()) {
+            page->client().page_did_finish_loading(url());
+        }
+    }
 }
 
 Page* Document::page()