Mojo C++ bindings: support enum validation.

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

 {%- import "validation_macros.tmpl" as validation_macros %}
 {%- set class_name = union.name ~ "_Data" %}
 {%- set enum_name = union.name ~ "_Tag" -%}
 
 // static
 {{class_name}}* {{class_name}}::New(mojo::internal::Buffer* buf) {
   return new (buf->Allocate(sizeof({{class_name}}))) {{class_name}}();
 }
 
 // static
@@ skipping 16 matching lines @@
     ReportValidationError(
         mojo::internal::VALIDATION_ERROR_ILLEGAL_MEMORY_RANGE);
     return false;
   }
   const {{class_name}}* object = static_cast<const {{class_name}}*>(data);
   MOJO_ALLOW_UNUSED_LOCAL(object);
 
   switch (object->tag) {
 {%  for field in union.fields %}
     case {{enum_name}}::{{field.name|upper}}: {
-{{ validation_macros.validate_union_field(field, union)|indent(8) }}
+      {{ validation_macros.validate_union_field(field, union)|indent(6) }}
     }
 {%- endfor %}
     default: {
       ReportValidationError(
           mojo::internal::VALIDATION_ERROR_UNKNOWN_UNION_TAG,
           "unknown tag in {{union.name}}");
       return false;
     }
   }
 }
@@ skipping 39 matching lines @@
 {%-   elif field.kind|is_interface_kind %}
       mojo::internal::DecodeHandle(
           reinterpret_cast<mojo::internal::Interface_Data*>(
               &data.f_{{field.name}}), handles);
 {%-   endif %}
       return;
     }
 {%- endfor %}
   }
 }