[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.
 ---#}
 {%- macro enum_decl(enum) %}
+{%- 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 %}
+{%-   for field in enum.fields %}
+{%-      if field.value %}
   {{field.name}} = {{field.value|expression_to_text}},
-{%-    else %}
+{%-      else %}
   {{field.name}},
-{%-    endif %}
-{%- endfor %}
+{%-      endif %}
+{%-   endfor %}
 };
+{%- endif %}
 {%- endmacro %}
 
 {%- macro enum_data_decl(enum) %}
 struct {{enum.name}}_Data {
  public:
   // Used to identify Mojom Enum Data Classes.
   typedef void MojomEnumDataType;
 
   static bool const kIsExtensible = {% if enum.extensible %}true{% else %}false{% endif %};
 
   static bool IsKnownValue(int32_t value) {
+{%- if enum.native_only %}
+    return mojo::EnumTraits<{{enum|cpp_wrapper_type}}>::IsKnownValue(value);
+{%- else %}
     switch (value) {
-{%- for enum_field in enum.fields|groupby('numeric_value') %}
+{%-   for enum_field in enum.fields|groupby('numeric_value') %}
       case {{enum_field[0]}}:
-{%- endfor %}
+{%-   endfor %}
         return true;
     }
     return false;
+{%- endif %}
   }
 
   int32_t value;
 };
 {%- endmacro %}
 
 {#--- macros for enum-associated functions. Namely:
   * operator<<(): outputs the given enum value.
   * IsKnownEnumValue(): returns true if the given enum value exists in this
         generated version of enum.
 ---#}
 
 {%- macro enum_stream_operator(enum) %}
+{%- if not enum.native_only %}
 inline std::ostream& operator<<(std::ostream& os, {{enum|get_name_for_kind}} value) {
   switch(value) {
-{%- for _, values in enum.fields|groupby('numeric_value') %}
+{%-   for _, values in enum.fields|groupby('numeric_value') %}
     case {{enum|get_name_for_kind}}::{{values[0].name}}:
       return os << "{{enum|get_name_for_kind}}::
-{%-   if values|length > 1 -%}
+{%-     if values|length > 1 -%}
       {{'{'}}
-{%-   endif -%}
+{%-     endif -%}
       {{values|map(attribute='name')|join(', ')}}
-{%-   if values|length > 1 -%}
+{%-     if values|length > 1 -%}
       {{'}'}}
-{%-   endif -%}
+{%-     endif -%}
       ";
-{%- endfor %}
+{%-   endfor %}
     default:
       return os << "Unknown {{enum|get_name_for_kind}} value: " << static_cast<int32_t>(value);
   }
 }
+{%- endif %}
 {%- endmacro %}
 
 {%- macro is_known_enum_value(enum) %}
 inline bool IsKnownEnumValue({{enum|get_name_for_kind}} value) {
   return {{enum|get_qualified_name_for_kind(internal=True)}}::IsKnownValue(
       static_cast<int32_t>(value));
 }
 {%- endmacro %}