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

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);
+  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);
 
 {% from "enum_macros.tmpl" import enum_data_decl -%}
 {#--- Enums #}
 {%- for enum in struct.enums -%}
 {%-   if enum|is_native_only_kind %}
   using {{enum.name}}_Data = mojo::internal::NativeEnum_Data;
 {%-   else %}
@@ skipping 28 matching lines @@
   uint8_t padfinal_[{{pad}}];
 {%-   endif %}
 {%- endif %}
 
  private:
   {{class_name}}();
   ~{{class_name}}() = delete;
 };
 static_assert(sizeof({{class_name}}) == {{struct.versions[-1].num_bytes}},
               "Bad sizeof({{class_name}})");