IDL: Generate trace() method in union container types when required

Source/bindings/templates/union.h

Index: Source/bindings/templates/union.h
diff --git a/Source/bindings/templates/union.h b/Source/bindings/templates/union.h
index 651b64757916d250b0e8e4f8a12d5b0f1c6512b0..8db42b44440da1776484f0ab3943b7d77f82b09d 100644
--- a/Source/bindings/templates/union.h
+++ b/Source/bindings/templates/union.h
@@ -30,6 +30,10 @@ public:
     void set{{member.type_name}}({{member.rvalue_cpp_type}});
 
     {% endfor %}
+    {% if container.needs_trace %}
+    void trace(Visitor*);
+
+    {% endif %}
 private:
     enum SpecificTypes {
         SpecificTypeNone,