LibWeb: Update HTMLObjectElement's children on fallback state changes

This is another event upon which the task to determine an object's
respresentation must be queued:

    * one of the element's ancestor object elements changes to or from
      showing its fallback content

For example, on Acid2, the image for the eyes is nested below an object
that is designed to fail to load. This ensures the eyes will render as
the fallback of the failed parent object.

Userland/Libraries/LibWeb/HTML/HTMLObjectElement.h

@@ -42,6 +42,7 @@ private:
     void run_object_representation_fallback_steps();
 
     void convert_resource_to_image();
+    void update_layout_and_child_objects();
 
     // ^ResourceClient
     virtual void resource_did_load() override;