Implement unions as members of structs.

mojo/public/tools/bindings/generators/cpp_templates/wrapper_class_definition.tmpl

 // static
 {{struct.name}}Ptr {{struct.name}}::New() {
   {{struct.name}}Ptr rv;
   mojo::internal::StructHelper<{{struct.name}}>::Initialize(&rv);
   return rv.Pass();
 }
 
 {{struct.name}}::{{struct.name}}()
 {%- for field in struct.fields %}
     {% if loop.first %}:{% else %} {% endif %} {{field.name}}({{field|default_value}}){% if not loop.last %},{% endif %}
 {%- endfor %} {
 }
 
 {{struct.name}}::~{{struct.name}}() {
 }
 
 {%  if struct|is_cloneable_kind %}
 {{struct.name}}Ptr {{struct.name}}::Clone() const {
   {{struct.name}}Ptr rv(New());
 {%-   for field in struct.fields %}
-{%-     if field.kind|is_struct_kind or field.kind|is_array_kind or field.kind|is_map_kind %}
+{%-     if field.kind|is_object_kind and not field.kind|is_string_kind %}
   rv->{{field.name}} = {{field.name}}.Clone();
 {%-     else %}
   rv->{{field.name}} = {{field.name}};
 {%-     endif %}
 {%-   endfor %}
   return rv.Pass();
 }
 {%  endif %}
 
 bool {{struct.name}}::Equals(const {{struct.name}}& other) const {
 {%-  for field in struct.fields %}
   if (!mojo::internal::ValueTraits<{{field.kind|cpp_wrapper_type}}>::Equals({{field.name}}, other.{{field.name}}))
     return false;
 {%-  endfor %}
   return true;
 }