GitLab

Executing some very badly needed clean-up of code that handles passing
around round trip time measurements and managing stats.  This change
removes a lot of dead code, addresses TODOs for minor things that were
dependent on this clean-up, and enhances RTCP unit testing.

BUG=404813

Review URL: https://codereview.chromium.org/532373003

Cr-Commit-Position: refs/heads/master@{#293887}