mojo: Allow circular dependencies between structs

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

 {%- macro validate(struct, class_name) %}
   if (!data)
     return true;
 
   if (!ValidateStructHeader(
           data, sizeof({{class_name}}),
           {{struct.packed.packed_fields|length}}, bounds_checker)) {
     return false;
   }
 
@@ skipping 20 matching lines @@
 {%-       if kind|is_any_array_kind or kind|is_string_kind %}
   if (!{{wrapper_type}}::Data_::Validate<
           {{kind|get_array_validate_params|indent(10)}}>(
               mojo::internal::DecodePointerRaw(&object->{{name}}.offset),
               bounds_checker)) {
 {%-       elif kind|is_map_kind %}
   if (!{{wrapper_type}}::Data_::Validate<
           {{kind.value_kind|get_map_validate_params|indent(10)}}>(
               mojo::internal::DecodePointerRaw(&object->{{name}}.offset),
               bounds_checker)) {
+{%-       elif kind|is_struct_kind %}
+  if (!{{kind|get_name_for_kind}}::Data_::Validate(
+          mojo::internal::DecodePointerRaw(&object->{{name}}.offset),
+          bounds_checker)) {
 {%-       else %}
   if (!{{wrapper_type}}::Data_::Validate(
           mojo::internal::DecodePointerRaw(&object->{{name}}.offset),
           bounds_checker)) {
 {%-       endif %}
     return false;
   }
 {%-     elif kind|is_any_handle_kind %}
 {%-       if not kind|is_nullable_kind %}
   if (object->{{name}}.value() == mojo::internal::kEncodedInvalidHandleValue) {
@@ skipping 60 matching lines @@
 
 {%- macro decodes(struct) -%}
 {%-   for pf in struct.packed.packed_fields %}
 {%-     if pf.field.kind|is_object_kind %}
 mojo::internal::Decode(&{{pf.field.name}}, handles);
 {%-     elif pf.field.kind|is_any_handle_kind %}
 mojo::internal::DecodeHandle(&{{pf.field.name}}, handles);
 {%-     endif %}
 {%-   endfor %}
 {%- endmacro -%}