LibWeb: Wait until child navigable's SHE is ready before navigating

This change fixes a bug that can be reproduced with the following steps:
```js
const iframe = document.createElement("iframe");
document.body.appendChild(iframe);
iframe.contentWindow.location.href = ("http://localhost:8080/demo.html");
```

These steps are executed in the following order:
1. Create iframe and schedule session history traversal task that adds
   session history entry for the iframe.
2. Generate navigation id for scheduled navigation to
   `http://localhost:8080/demo.html`.
3. Execute the scheduled session history traversal task, which adds
   session history entry for the iframe.
4. Ooops, navigation to `http://localhost:8080/demo.html` is aborted
   because addings SHE for the iframe resets the navigation id.

This change fixes this by delaying all navigations until SHE for a
navigable is created.

Libraries/LibWeb/HTML/HTMLObjectElement.cpp

@@ -446,7 +446,7 @@ void HTMLObjectElement::run_object_representation_handler_steps(Fetch::Infrastru
         // If the object element's content navigable is null, then create a new child navigable for the element.
         if (!m_content_navigable && in_a_document_tree()) {
             MUST(create_new_child_navigable());
-            set_content_navigable_initialized();
+            set_content_navigable_has_session_history_entry_and_ready_for_navigation();
         }
 
         // NOTE: Creating a new nested browsing context can fail if the document is not attached to a browsing context