mojo bindings: Support enums with no values

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

 {#---
   Macro for enum definition, and the declaration of associated functions.
 ---#}
 
 {%- macro enum_decl(enum) %}
 {%-   set enum_name = enum|get_name_for_kind(flatten_nested_kind=True) %}
 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 %}
 };
 
 inline std::ostream& operator<<(std::ostream& os, {{enum_name}} value) {
+{%- if enum.fields %}
   switch(value) {
 {%-   for _, values in enum.fields|groupby('numeric_value') %}
     case {{enum_name}}::{{values[0].name}}:
       return os << "{{enum_name}}::
 {%-     if values|length > 1 -%}
       {{'{'}}
 {%-     endif -%}
       {{values|map(attribute='name')|join(', ')}}
 {%-     if values|length > 1 -%}
       {{'}'}}
 {%-     endif -%}
       ";
 {%-   endfor %}
     default:
       return os << "Unknown {{enum_name}} value: " << static_cast<int32_t>(value);
   }
+{%- else %}
+  return os << "Unknown {{enum_name}} value: " << static_cast<int32_t>(value);
+{%- endif %}
 }
 
 {#-   Returns true if the given enum value exists in this version of enum. #}
 inline bool IsKnownEnumValue({{enum_name}} value) {
   return {{enum|get_name_for_kind(internal=True,
                                   flatten_nested_kind=True)}}::IsKnownValue(
       static_cast<int32_t>(value));
 }
 {%- endmacro %}
 
@@ skipping 74 matching lines @@
   }
   static void constructDeletedValue({{enum_name}}& slot, bool) {
     slot = static_cast<{{enum_name}}>({{deleted_value}});
   }
   static bool isDeletedValue(const {{enum_name}}& value) {
     return value == static_cast<{{enum_name}}>({{deleted_value}});
   }
 };
 }  // namespace WTF
 {%- endmacro %}