GitLab

Temporary measure until both Pipeline is reworked to provide a time
estimate to the main thread and VideoRendererImpl's internal thread
is eliminated.

This is similar to the the locking currently present in
AudioRendererImpl.

BUG=412764

Review URL: https://codereview.chromium.org/562673003

Cr-Commit-Position: refs/heads/master@{#294207}