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

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

 {%- set class_name = union.name ~ "_Data" -%}
 {%- set enum_name = union.name ~ "_Tag" -%}
 {%- import "struct_macros.tmpl" as struct_macros %}
 
 class {{class_name}} {
  public:
   // Used to identify Mojom Union Data Classes.
   typedef void MojomUnionDataType;
-  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}}();
+  }
+
   {{class_name}}();
   // Do nothing in the destructor since it won't be called.
   ~{{class_name}}() {}
 
   static bool Validate(const void* data,
                        mojo::internal::ValidationContext* validation_context,
                        bool inlined);
 
   bool is_null() const {
     return size == 0;
@@ skipping 20 matching lines @@
 {%- endfor %}
     uint64_t unknown;
   };
 
   uint32_t size;
   {{enum_name}} tag;
   Union_ data;
 };
 static_assert(sizeof({{class_name}}) == mojo::internal::kUnionDataSize,
               "Bad sizeof({{class_name}})");