IDL: Use Nullable for union type return value

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 278 matching lines @@
 {# Length check needed to skip action on legacy calls without enough arguments.
    http://crbug.com/353484 #}
 if (info.Length() >= 2 && listener && !impl->toNode())
     {{hidden_dependency_action}}(info.Holder(), info[1], {{v8_class}}::eventListenerCacheIndex, info.GetIsolate());
 {% endif %}
 {% endmacro %}
 
 
 {######################################}
 {% macro union_type_method_call_and_set_return_value(method) %}
-{% for cpp_type in method.cpp_type %}
-bool result{{loop.index0}}Enabled = false;
-{{cpp_type}} result{{loop.index0}};
+{% for argument in method.union_arguments %}
+{{argument.cpp_type}} {{argument.cpp_value}};
 {% endfor %}
 {{method.cpp_value}};
 {% if method.is_null_expression %}{# used by getters #}
 if ({{method.is_null_expression}})
     return;
 {% endif %}
-{% for v8_set_return_value in method.v8_set_return_value %}
-if (result{{loop.index0}}Enabled) {
-    {{v8_set_return_value}};
+{% for argument in method.union_arguments %}
+if ({{argument.null_check_value}}) {
+    {{argument.v8_set_return_value}};
     return;
 }
 {% endfor %}
 {# Fall back to null if none of the union members results are returned #}
+{% if method.is_null_expression %}
+ASSERT_NOT_REACHED();
+{% else %}
 v8SetReturnValueNull(info);
+{% endif %}
 {% endmacro %}
 
 
 {######################################}
 {% macro throw_type_error(method, error_message) %}
 {% if method.has_exception_state %}
 exceptionState.throwTypeError({{error_message}});
 {{throw_from_exception_state(method)}};
 {% elif method.idl_type == 'Promise' %}
 v8SetReturnValue(info, ScriptPromise::rejectRaw(info.GetIsolate(), V8ThrowException::createTypeError({{type_error_message(method, error_message)}}, info.GetIsolate())));
@@ skipping 290 matching lines @@
 v8::Handle<v8::Object> wrapper = wrap(impl.get(), info.Holder(), info.GetIsolate());
 {% else %}
 {% set constructor_class = v8_class + ('Constructor'
                                        if constructor.is_named_constructor else
                                        '') %}
 v8::Handle<v8::Object> wrapper = info.Holder();
 V8DOMWrapper::associateObjectWithWrapper<{{v8_class}}>(impl.release(), &{{constructor_class}}::wrapperTypeInfo, wrapper, info.GetIsolate(), {{wrapper_configuration}});
 {% endif %}
 v8SetReturnValue(info, wrapper);
 {% endmacro %}