When in DRP holdback group for HTTP page loads, Chrome should not:
(i) Record DRP page load metrics UMA
(ii) Send pingback to the server.

This is because in Control group, Chrome tries to guess if it would have used DRP if it was in Enabled group. If so, it records metrics for that page load, and sends pingback. This leads to biased dataset (see http://shortn/_lDL6IX6W6Z for details).

Instead of using PageLoad.Clients.DataReductionProxy.* UMA, we can use PageLoad.Clients.Scheme.Http.* UMA which provides similar data but is not subject to bias (See Issue 850736).