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

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

 {%- import "validation_macros.tmpl" as validation_macros %}
 {%- set class_name = struct.name ~ "_Data" %}
 
 // 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) {
   if (!data)
     return true;
 
   if (!ValidateStructHeaderAndClaimMemory(data, validation_context))
     return false;
 
   // NOTE: The memory backing |object| may be smaller than |sizeof(*object)| if
@@ skipping 50 matching lines @@
 {%-   endif %}
 {%- endfor %}
 
   return true;
 }
 
 {{class_name}}::{{class_name}}() {
   header_.num_bytes = sizeof(*this);
   header_.version = {{struct.versions[-1].version}};
 }