Mojo: Simplify object serialization

mojo/public/bindings/generators/cpp_templates/params_serialization.tmpl

-{%- import "struct_macros.tmpl" as struct_macros %}
-{%- set class_name = "%s::internal::%s_Data"|format(namespace, struct.name) %}
-{%- set param_name = "params" %}
-template <>
-class ObjectTraits<{{class_name}}> {
- public:
-  static void CloseHandles({{class_name}}* {{param_name}}) {
-{%- for pf in struct.packed.packed_fields if pf.field.kind|is_object_kind %}
-    mojo::internal::CloseHandles({{param_name}}->{{pf.field.name}}_.ptr);
-{%- endfor %}
-  }
-
-  static void EncodePointersAndHandles(
-      {{class_name}}* {{param_name}},
-      std::vector<Handle>* handles) {
-    {{ struct_macros.encodes(struct, param_name)|indent(4) }}
-  }
-
-  static bool DecodePointersAndHandles(
-      {{class_name}}* {{param_name}},
-      Message* message) {
-    {{ struct_macros.decodes(struct, param_name)|indent(4) }}
-    return true;
-  }
-};