[mojo] Add pickling support for native-only structs

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

 {%- import "interface_macros.tmpl" as interface_macros %}
 {%- import "struct_macros.tmpl" as struct_macros %}
 {%- from "enum_macros.tmpl" import is_valid_enum_def %}
 
 {%- set class_name = interface.name %}
 {%- set proxy_name = interface.name ~ "Proxy" %}
 {%- set namespace_as_string = "%s"|format(namespace|replace(".","::")) %}
 
 {%- macro alloc_params(struct, serialization_context) %}
   bool success = true;
 {%-   for param in struct.packed.packed_fields_in_ordinal_order %}
-  {{param.field.kind|cpp_result_type}} p_{{param.field.name}}{};
+  {{param.field.kind|cpp_wrapper_type}} p_{{param.field.name}}{};
 {%-   endfor %}
   {{struct_macros.deserialize(struct, "params", "p_%s", serialization_context, "success")}}
   if (!success)
     return false;
 {%- endmacro %}
 
 {%- macro pass_params(parameters) %}
 {%-   for param in parameters %}
 {%-     if param.kind|is_move_only_kind -%}
 std::move(p_{{param.name}})
@@ skipping 297 matching lines @@
     default:
       break;
   }
 
   // Unrecognized message.
   ReportValidationError(
       mojo::internal::VALIDATION_ERROR_MESSAGE_HEADER_UNKNOWN_METHOD);
   return false;
 }
 {%- endif -%}