Protect DefaultVideoQualityAnalyzer::peers_ with lock, because it's now accessed from multiple threads. Bug: webrtc:12247 Change-Id: I41932afe678979f6da9e8d0d5fe2e1ffef0fb513 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/203880 Reviewed-by: Andrey Logvin <landrey@webrtc.org> Commit-Queue: Artem Titov <titovartem@webrtc.org> Cr-Commit-Position: refs/heads/master@{#33073}