LibWeb: Add an interface to be notified of Document state changes

Some HTML elements, e.g. HTMLMediaElement, need to take action when the
document becomes inactive.

Userland/Libraries/LibWeb/Forward.h

@@ -189,6 +189,7 @@ class CustomEvent;
 class Document;
 class DocumentFragment;
 class DocumentLoadEventDelayer;
+class DocumentObserver;
 class DocumentType;
 class DOMEventListener;
 class DOMImplementation;