LibWeb: Run the object representation task when the active state changes

Currently, the following JS snippet will hang indefinitely:

    new DOMParser().parseFromString("<object>", "text/html");

Because the document into which the object is inserted is not active. So
the task queued to run the representation steps will never run.

This patch implements the spec steps to rerun the representation steps
when the active state changes, and avoid the hang when the object is
created in an inactive document.

Libraries/LibWeb/HTML/HTMLObjectElement.h

@@ -90,6 +90,8 @@ private:
 
     GC::Ptr<SharedResourceRequest> m_resource_request;
 
+    GC::Ptr<DOM::DocumentObserver> m_document_observer;
+
     Optional<DOM::DocumentLoadEventDelayer> m_document_load_event_delayer_for_object_representation_task;
     Optional<DOM::DocumentLoadEventDelayer> m_document_load_event_delayer_for_resource_load;
 };