Make mojo::internal::Buffer a non-virtual class.

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
 bool {{class_name}}::Validate(
     const void* data,
     mojo::internal::ValidationContext* validation_context,
     bool inlined) {
   if (!data)
     return true;
 
   if (!ValidateUnionHeaderAndClaimMemory(data, inlined, validation_context))
     return false;
 
@@ skipping 20 matching lines @@
 
 void {{class_name}}::set_null() {
   size = 0U;
   tag = static_cast<{{enum_name}}>(0);
   data.unknown = 0U;
 }
 
 {{class_name}}::{{class_name}}() {
 }