Mojo C++ bindings: extract code shared by different variants.

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

-{%- set mojom_type = enum|get_qualified_name_for_kind %}
+{%- set mojom_type = enum|get_qualified_name_for_kind(
+        flatten_nested_kind=True) %}
 
 template <>
 struct EnumTraits<{{mojom_type}}, {{mojom_type}}> {
   static {{mojom_type}} ToMojom({{mojom_type}} input) { return input; }
   static bool FromMojom({{mojom_type}} input, {{mojom_type}}* output) {
     *output = input;
     return true;
   }
 };
 
 namespace internal {
 
 template <typename MaybeConstUserType>
 struct Serializer<{{mojom_type}}, MaybeConstUserType> {
   using UserType = typename std::remove_const<MaybeConstUserType>::type;
   using Traits = EnumTraits<{{mojom_type}}, UserType>;
 
   static void Serialize(UserType input, int32_t* output) {
     *output = static_cast<int32_t>(Traits::ToMojom(input));
   }
 
   static bool Deserialize(int32_t input, UserType* output) {
     return Traits::FromMojom(static_cast<{{mojom_type}}>(input), output);
   }
 };
 
 }  // namespace internal