Mojo: more idiomatic C++ bindings

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

+size_t GetSerializedSize_(const {{struct.name}}Ptr& input) {
+  size_t size = sizeof(internal::{{struct.name}}_Data);
+{%- for pf in struct.packed.packed_fields if pf.field.kind|is_object_kind %}
+  if (input->{{pf.field.name}})
+    size += GetSerializedSize_(input->{{pf.field.name}});
+{%- endfor %}
+  return size;
+}
+
+void Serialize_({{struct.name}}Ptr input, mojo::Buffer* buf,
+                internal::{{struct.name}}_Data** output) {
+  internal::{{struct.name}}_Data* result =
+      internal::{{struct.name}}_Data::New(buf);
+{%- for pf in struct.packed.packed_fields if pf.field.kind|is_object_kind %}
+{%-   if pf.field.kind|is_object_kind %}
+  if (input->{{pf.field.name}}) {
+    {{pf.field.kind|cpp_type}} tmp;
+    Serialize_(mojo::internal::Forward(input->{{pf.field.name}}), buf, &tmp);
+    result->set_{{pf.field.name}}(tmp);
+  }
+{%-   else %}
+  result->set_{{pf.field.name}}(input->{{pf.field.name}});
+{%-   endif %}
+{%- endfor %}
+  *output = result;
+}
+
+void Deserialize_(const internal::{{struct.name}}_Data* input,
+                  {{struct.name}}Ptr* output) {
+  {{struct.name}}Ptr result({{struct.name}}::New());
+{%- for pf in struct.packed.packed_fields if pf.field.kind|is_object_kind %}
+{%-   if pf.field.kind|is_object_kind %}
+  if (input->{{pf.field.name}}())
+    Deserialize_(input->{{pf.field.name}}(), &result->{{pf.field.name}});
+{%-   else %}
+  result->{{pf.field.name}} = input->{{pf.field.name}}();
+{%-   endif %}
+{%- endfor %}
+  *output = result.Pass();
+}