[mojo] Support native types with mojom wire format

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

 {% from "enum_macros.tmpl" import enum_decl -%}
+
+class {{struct.name}}_Reader;
+
 class {{struct.name}} {
  public:
   using Data_ = internal::{{struct.name}}_Data;
+  using Reader = {{struct.name}}_Reader;
 
 {#--- Enums #}
 {%- for enum in struct.enums -%}
   {{enum_decl(enum, is_static=true)|indent(2)}}
 {%- endfor %}
 
 {#--- Constants #}
 {%- for constant in struct.constants %}
 {%-   if constant.kind|is_integral_kind %}
   static const {{constant.kind|cpp_pod_type}} {{constant.name}} = {{constant|constant_value}};
@@ skipping 22 matching lines @@
 {%- endif %}
   bool Equals(const {{struct.name}}& other) const;
 
 {#--- Struct members #}
 {%  for field in struct.fields %}
 {%-   set type = field.kind|cpp_wrapper_type %}
 {%-   set name = field.name %}
   {{type}} {{name}};
 {%- endfor %}
 };