[mojo] Support native types with mojom wire format

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

 {%- import "struct_macros.tmpl" as struct_macros %}
 size_t GetSerializedSize_(const {{struct.name}}Ptr& input) {
   if (!input)
     return 0;
   {{struct_macros.get_serialized_size(struct, "input->%s")}}
   return size;
 }
 
 void Serialize_({{struct.name}}Ptr input, mojo::internal::Buffer* buf,
                 internal::{{struct.name}}_Data** output) {
@@ skipping 11 matching lines @@
   bool success = true;
   if (input) {
     {{struct.name}}Ptr result({{struct.name}}::New());
     {{struct_macros.deserialize(struct, "input", "result->%s", "context", "success")|indent(2)}}
     *output = result.Pass();
   } else {
     output->reset();
   }
   return success;
 }
+
+{%- if not variant %}
+
+{{struct.name}}_Reader::{{struct.name}}_Reader(
+    internal::{{struct.name}}_Data*  data,
+    mojo::internal::SerializationContext* context)
+    : data_(data), context_(context) { }
+
+{%- for pf in struct.packed.packed_fields_in_ordinal_order %}
+{%-   set name = pf.field.name -%}
+{%-   set kind = pf.field.kind -%}
+{%-   if kind|is_struct_kind %}
+{{kind|get_name_for_kind}}_Reader {{struct.name}}_Reader::{{name}}() const {
+  return {{kind|get_name_for_kind}}_Reader(data_->{{name}}.ptr, context_);
+}
+{%-   endif %}
+{%- endfor %}
+
+{%- endif %}