[mojo-bindings] Support reuse of native enum classes

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

 {#---
   Macro for enum definition, and the declaration of associated functions.
   `is_static` is relevant if this enum declaration is defined within a class, in
   which case associated functions need to be static.
 ---#}
 {%- macro enum_decl(enum, is_static=false) %}
+{%- if enum.native_only %}
+using {{enum.name}} = {{enum|cpp_wrapper_type}};
+{%- else %}
 enum class {{enum.name}} : int32_t {
 {%- for field in enum.fields %}
 {%-    if field.value %}
   {{field.name}} = {{field.value|expression_to_text}},
 {%-    else %}
   {{field.name}},
 {%-    endif %}
 {%- endfor %}
 };
 {{is_valid_enum_decl(enum, is_static)}}
+{% endif %}
 {%- endmacro %}
 
 {#--- macros for the declaration & definitions of enum-associated functions.
       Namely:
   * {enum_name}_IsValidValue:  returns true if the given enum has a valid value
                                for this generated version of enum.
 ---#}
 
 {%- macro is_valid_enum_decl(enum, is_static=false) %}
 {% if is_static %}static {% endif -%}
@@ skipping 31 matching lines @@
 {{enum.name}}_IsValidValue({{enum.name}} value) {
   switch (static_cast<int32_t>(value)) {
 {%- for enum_field in enum.fields|groupby('numeric_value') %}
     case {{enum_field[0]}}:
 {%- endfor %}
       return true;
   }
   return false;
 }
 {%- endmacro %}