LibWeb/IDB: Implement recent spec changes

Author: https://github.com/stelar7
Commit: 9d5f6108e4
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/4620
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/IndexedDB/Internal/Index.h

@@ -41,6 +41,8 @@ public:
 
     [[nodiscard]] bool has_record_with_key(GC::Ref<Key> key);
 
+    HTML::SerializationRecord referenced_value(IndexRecord const& index_record) const;
+
 protected:
     virtual void visit_edges(Visitor&) override;