IDL: Support optional union type arguments with default values

Source/bindings/templates/methods.cpp

 {##############################################################################}
 {% macro generate_method(method, world_suffix) %}
 {% filter conditional(method.conditional_string) %}
 static void {{method.name}}{{method.overload_index}}Method{{world_suffix}}(const v8::FunctionCallbackInfo<v8::Value>& info)
 {
     {# Local variables #}
     {% if method.has_exception_state %}
     ExceptionState exceptionState(ExceptionState::ExecutionContext, "{{method.name}}", "{{interface_name}}", info.Holder(), info.GetIsolate());
     {% endif %}
     {# Overloaded methods have length checked during overload resolution #}
@@ skipping 46 matching lines @@
 {% endmacro %}
 
 
 {######################################}
 {% macro generate_arguments(method, world_suffix) %}
 {% for argument in method.arguments %}
 {{generate_argument_var_declaration(argument)}};
 {% endfor %}
 {
     {% for argument in method.arguments %}
-    {% if argument.default_value %}
+    {% if argument.set_default_value %}
     if (!info[{{argument.index}}]->IsUndefined()) {
         {{generate_argument(method, argument, world_suffix) | indent(8)}}
     } else {
-        {{argument.name}} = {{argument.default_value}};
+        {{argument.set_default_value}};
     }
     {% else %}
     {{generate_argument(method, argument, world_suffix) | indent}}
     {% endif %}
     {% endfor %}
 }
 {% endmacro %}
 
 
 {######################################}
@@ skipping 547 matching lines @@
     {% for method in conditionally_enabled_methods %}
     {% filter per_context_enabled(method.per_context_enabled_function) %}
     {% filter exposed(method.exposed_test) %}
     prototypeObject->Set(v8AtomicString(isolate, "{{method.name}}"), v8::FunctionTemplate::New(isolate, {{cpp_class_or_partial}}V8Internal::{{method.name}}MethodCallback, v8Undefined(), defaultSignature, {{method.number_of_required_arguments}})->GetFunction());
     {% endfilter %}
     {% endfilter %}
     {% endfor %}
     {% endif %}
 }
 {%- endmacro %}