Mojo C++ bindings: support enum validation.

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

 size_t GetSerializedSize_(const {{union.name}}Ptr& input, bool inlined) {
   size_t size = 0U;
   if (!inlined) {
     size += sizeof(internal::{{union.name}}_Data);
   }
 
   if (!input)
     return size;
 
   mojo::internal::UnionAccessor<{{union.name}}> input_acc(input.get());
@@ skipping 61 matching lines @@
             input_acc.data()->{{field.name}}->release().value();
 {%    elif field.kind|is_interface_kind %}
         mojo::internal::Interface_Data* {{field.name}} =
             reinterpret_cast<mojo::internal::Interface_Data*>(
                 &result->data.f_{{field.name}});
         mojo::internal::InterfacePointerToData(
             std::move(*input_acc.data()->{{field.name}}), {{field.name}});
 {%-   elif field.kind|is_associated_kind %}
         // TODO(yzshen): add seralization logic for associated kinds.
 {%    elif field.kind|is_enum_kind %}
-        result->data.f_{{field.name}} = static_cast<int32_t>(input_acc.data()->{{field.name}});
+        result->data.f_{{field.name}}.value = static_cast<int32_t>(input_acc.data()->{{field.name}});
 {%    else %}
         result->data.f_{{field.name}} = input_acc.data()->{{field.name}};
 {%-   endif %}
         break;
       }
 {%- endfor %}
     }
   } else if (inlined) {
     result->set_null();
   } else {
@@ skipping 24 matching lines @@
         {{field.kind|cpp_wrapper_type}} {{field.name}}_out;
         mojo::internal::Interface_Data* {{field.name}}_in =
             reinterpret_cast<mojo::internal::Interface_Data*>(
                 &input->data.f_{{field.name}});
         mojo::internal::InterfaceDataToPointer(
             {{field.name}}_in, &{{field.name}}_out);
         result->set_{{field.name}}(std::move({{field.name}}_out));
 {%-   elif field.kind|is_associated_kind %}
         // TODO(yzshen): add deserialization logic for associated kinds.
 {%    elif field.kind|is_enum_kind %}
-        result->set_{{field.name}}(static_cast<{{field.kind|cpp_wrapper_type}}>(input->data.f_{{field.name}}));
+        result->set_{{field.name}}(static_cast<{{field.kind|cpp_wrapper_type}}>(input->data.f_{{field.name}}.value));
 {%    else %}
         result->set_{{field.name}}(input->data.f_{{field.name}});
 {%-   endif %}
         break;
       }
 {%- endfor %}
     }
     *output = std::move(result);
   } else {
     output->reset();
   }
   return success;
 }