IDL: Generate union type containers

Source/bindings/templates/interface.cpp

 {% extends 'interface_base.cpp' %}
 
 
 {##############################################################################}
 {% block indexed_property_getter %}
 {% if indexed_property_getter and not indexed_property_getter.is_custom %}
 {% set getter = indexed_property_getter %}
 static void indexedPropertyGetter(uint32_t index, const v8::PropertyCallbackInfo<v8::Value>& info)
 {
     {{cpp_class}}* impl = {{v8_class}}::toImpl(info.Holder());
@@ skipping 150 matching lines @@
     if (!info.Holder()->GetRealNamedPropertyInPrototypeChain(name).IsEmpty())
         return;
 
     {% endif %}
     {{cpp_class}}* impl = {{v8_class}}::toImpl(info.Holder());
     AtomicString propertyName = toCoreAtomicString(name);
     {% if getter.is_raises_exception %}
     v8::String::Utf8Value namedProperty(name);
     ExceptionState exceptionState(ExceptionState::GetterContext, *namedProperty, "{{interface_name}}", info.Holder(), info.GetIsolate());
     {% endif %}
-    {% if getter.union_arguments %}
+    {% if getter.union_result_members %}
     {{union_type_method_call_and_set_return_value(getter) | indent}}
     {% else %}
     {{getter.cpp_type}} result = {{getter.cpp_value}};
     {% if getter.is_raises_exception %}
     if (exceptionState.throwIfNeeded())
         return;
     {% endif %}
     if ({{getter.is_null_expression}})
         return;
     {{getter.v8_set_return_value}};
@@ skipping 839 matching lines @@
 }
 
 {% for method in methods if method.overloads and method.overloads.has_partial_overloads %}
 void {{v8_class}}::register{{method.name | blink_capitalize}}MethodForPartialInterface(void (*method)(const v8::FunctionCallbackInfo<v8::Value>&))
 {
     {{cpp_class}}V8Internal::{{method.name}}MethodForPartialInterface = method;
 }
 {% endfor %}
 {% endif %}
 {% endblock %}