IDL: Union type support for attributes

Source/bindings/templates/attributes.cpp

Index: Source/bindings/templates/attributes.cpp
diff --git a/Source/bindings/templates/attributes.cpp b/Source/bindings/templates/attributes.cpp
index 42b58a45e3419f55cd822ecbb8c18f6ff9aef106..698ed73cf5a8cc9647be822b3b654aa0ac3bf9e2 100644
--- a/Source/bindings/templates/attributes.cpp
+++ b/Source/bindings/templates/attributes.cpp
@@ -95,6 +95,10 @@ const v8::PropertyCallbackInfo<v8::Value>& info
     {% elif world_suffix %}
     {{attribute.v8_set_return_value_for_main_world}};
     {% else %}
+    {% if attribute.use_output_parameter_for_result %}
+    {{attribute.cpp_type}} result;
+    {{attribute.cpp_value}};
+    {% endif %}
     {{attribute.v8_set_return_value}};
     {% endif %}
 }
@@ -223,6 +227,9 @@ v8::Local<v8::Value> v8Value, const v8::PropertyCallbackInfo<void>& info
         return;
     }
     {% endif %}
+    {% if attribute.use_output_parameter_for_result %}
+    {{attribute.cpp_type}} cppValue;
+    {% endif %}
     {# impl #}
     {% if attribute.put_forwards %}
     {{cpp_class}}* proxyImpl = {{v8_class}}::toImpl(holder);