Improve gfx::Transform StructTraits.

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

 {%- set class_name = struct.name ~ "_Data" -%}
 
 class {{class_name}} {
  public:
   static {{class_name}}* New(mojo::internal::Buffer* buf) {
     return new (buf->Allocate(sizeof({{class_name}}))) {{class_name}}();
   }
 
   static bool Validate(const void* data,
                        mojo::internal::ValidationContext* validation_context);
@@ skipping 30 matching lines @@
 {%- if num_fields > 0 %}
 {%-   set last_field = struct.packed.packed_fields[num_fields - 1] %}
 {%-   set offset = last_field.offset + last_field.size %}
 {%-   set pad = offset|get_pad(8) %}
 {%-   if pad > 0 %}
   uint8_t padfinal_[{{pad}}];
 {%-   endif %}
 {%- endif %}
 
  private:
-  {{class_name}}();
+  {{class_name}}() : header_({sizeof(*this), {{struct.versions[-1].version}}}) {
+  }
   ~{{class_name}}() = delete;
 };
 static_assert(sizeof({{class_name}}) == {{struct.versions[-1].num_bytes}},
               "Bad sizeof({{class_name}})");