Adding locking to webrtc::voe::Channel to fix race conditions

Some members are accessed from the video processing thread for the
VoEVideoSync interface, and thus need to be protected. This is a
problem that TSan sometimes reports.

Also moved UpdatePlayoutTimestamp to private section since
it's only needed internally. And renamed least_required_delay_ms
to LeastRequiredDelayMs, since it no longer just returns a cached
value.

BUG=webrtc:4663

Review URL: https://codereview.webrtc.org/1263223002

Cr-Commit-Position: refs/heads/master@{#9706}
3 files changed