LibWeb: Start fleshing out update document for history step application

Author: https://github.com/kalenikaliaksandr
Commit: 699ead0939
Pull-request: https://github.com/SerenityOS/serenity/pull/21209
Issue: https://github.com/SerenityOS/serenity/issues/21185
Reviewed-by: https://github.com/ADKaster ✅

Userland/Libraries/LibWeb/HTML/TraversableNavigable.h

@@ -34,8 +34,8 @@ public:
     void set_system_visibility_state(VisibilityState);
 
     struct HistoryObjectLengthAndIndex {
-        size_t script_history_length;
-        size_t script_history_index;
+        u64 script_history_length;
+        u64 script_history_index;
     };
     HistoryObjectLengthAndIndex get_the_history_object_length_and_index(int) const;