LibWeb: Move serialization of serializable object's interface name

To be consistent with the deserialization steps, move serialization of
the serializable object's interface name out of the serialization steps.

Userland/Libraries/LibWeb/HTML/StructuredSerialize.cpp

@@ -306,6 +306,8 @@ public:
 
             m_serialized.append(ValueTag::SerializableObject);
 
+            TRY(serialize_string(m_vm, m_serialized, serializable.interface_name()));
+
             // 1. Perform the serialization steps for value's primary interface, given value, serialized, and forStorage.
             TRY(serializable.serialization_steps(m_serialized, m_for_storage));
         }