adding 30% drift to media generator (e.g. audio frame generated every 7ms instead of promised 10ms) works fine
adding 2% drift to video ntp-timestamp-stamper makes A/V sync fail.
BUG=webrtc:5504
R=pbos@webrtc.org,stefan@webrtc.org
Review URL: https://codereview.webrtc.org/1674413004
Cr-Commit-Position: refs/heads/master@{#11556}
