Mojo C++ bindings: add support for UnionTraits.

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
 bool {{class_name}}::Validate(
     const void* data,
     mojo::internal::ValidationContext* validation_context,
     bool inlined) {
   if (!data)
@@ skipping 15 matching lines @@
 {%- endfor %}
     default: {
       ReportValidationError(
           validation_context,
           mojo::internal::VALIDATION_ERROR_UNKNOWN_UNION_TAG,
           "unknown tag in {{union.name}}");
       return false;
     }
   }
 }
-
-void {{class_name}}::set_null() {
-  size = 0U;
-  tag = static_cast<{{enum_name}}>(0);
-  data.unknown = 0U;
-}
-
-{{class_name}}::{{class_name}}() {
-}
-