Switch base::Value typemapping to be by value instead of by unique_ptr.

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

Index: mojo/public/tools/bindings/generators/cpp_templates/struct_data_view_declaration.tmpl
diff --git a/mojo/public/tools/bindings/generators/cpp_templates/struct_data_view_declaration.tmpl b/mojo/public/tools/bindings/generators/cpp_templates/struct_data_view_declaration.tmpl
index 96e0d614d8ef59f635fb44f478bca29586b0dab5..25d245d10c99909177de7c98ed271453f423f11f 100644
--- a/mojo/public/tools/bindings/generators/cpp_templates/struct_data_view_declaration.tmpl
+++ b/mojo/public/tools/bindings/generators/cpp_templates/struct_data_view_declaration.tmpl
@@ -28,6 +28,9 @@ class {{struct.name}}DataView {
 {%-     else %}
     auto* pointer = &data_->{{name}};
 {%-     endif %}
+    if (pointer && pointer->is_null())
+      pointer = nullptr;
+
     return mojo::internal::Deserialize<{{kind|unmapped_type_for_serializer}}>(
         pointer, output, context_);
   }