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/HTMLIFrameElement.cpp

@@ -92,7 +92,8 @@ void HTMLIFrameElement::post_connection()
 
         // 3. Process the iframe attributes for insertedNode, with initialInsertion set to true.
         process_the_iframe_attributes(true);
-        set_content_navigable_initialized();
+
+        set_content_navigable_has_session_history_entry_and_ready_for_navigation();
     })));
 }