Mojo C++ bindings: support enum validation.

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

 size_t GetSerializedSize_(const {{struct.name}}Ptr& input);
 void Serialize_({{struct.name}}Ptr input, mojo::internal::Buffer* buffer,
                 internal::{{struct.name}}_Data** output);
 bool Deserialize_(internal::{{struct.name}}_Data* input,
                   {{struct.name}}Ptr* output,
                   mojo::internal::SerializationContext* context);
 
 {%- if not variant %}
 
 {# NOTE: Generated Reader instances are intentionally cheap to copy and devoid
@@ skipping 15 matching lines @@
 {%-     elif kind|is_any_handle_kind %}
   // TODO(rockot): Support reading handles. ({{name}}() omitted)
 {%-     elif kind|is_interface_kind %}
   // TODO(rockot): Support reading interfaces. ({{name}}() omitted)
 {%-     elif kind|is_interface_request_kind %}
   // TODO(rockot): Support reading interface requests. ({{name}}() omitted)
 {%-     elif kind|is_associated_kind %}
   // TODO(rockot): Support reading associated kinds. ({{name}}() omitted)
 {%-     elif kind|is_enum_kind %}
   {{kind|cpp_result_type}} {{name}}() const {
-    return static_cast<{{kind|cpp_result_type}}>(data_->{{name}});
+    return static_cast<{{kind|cpp_result_type}}>(data_->{{name}}.value);
   }
 {%-     else %}
   {{kind|cpp_result_type}} {{name}}() const { return data_->{{name}}; }
 {%-     endif %}
 {%-   endfor %}
 
  private:
   internal::{{struct.name}}_Data* data_;
   mojo::internal::SerializationContext* context_;
 };
@@ skipping 27 matching lines @@
 
   static bool Deserialize(internal::{{struct.name}}_Data* input,
                           NativeType* output,
                           mojo::internal::SerializationContext* context) {
     return mojo::StructTraits<{{struct.name}}, NativeType>::Read(
         {{struct.name}}_Reader(input, context), output);
   }
 };
 
 {%- endif %}  {# not variant #}