Add missing copy and move operator= and GetVariance and
GetStandardDeviation methods to the SamplesStatsCounter.
Change-Id: I02374aac23a00fdeefda16012311cd860bb4b1b5
Bug: webrtc:10138
Reviewed-on: https://webrtc-review.googlesource.com/c/121653
Commit-Queue: Artem Titov <titovartem@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26584}