For c++, Generate enum classes instead of enum from mojom.

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 60 matching lines @@
 {%-     endif %}
 {%    elif field.kind|is_any_handle_kind %}
         result->data.f_{{field.name}} =
             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(
             input_acc.data()->{{field.name}}->Pass(), {{field.name}});
+{%    elif field.kind|is_enum_kind %}
+        result->data.f_{{field.name}} = 
+          static_cast<int32_t>(input_acc.data()->{{field.name}});
 {%    else %}
         result->data.f_{{field.name}} = input_acc.data()->{{field.name}};
 {%-   endif %}
         break;
       }
 {%- endfor %}
       default:
         MOJO_CHECK(false) << "No sane way to serialize a union with an unknown tag.";
         break;
     }
@@ skipping 43 matching lines @@
 {%-   endif %}
         break;
       }
 {%- endfor %}
       default:
         MOJO_LOG(WARNING) << "Deserializing {{union.name}} with unknown tag!";
         // No way to deserialize the data when we encounter an unknown tag.
         break;
     }
   }
-}
+}