Use RaceChecker instead of ThreadChecker in remote_bitrate_estimator.

BUG=webrtc:7826

Review-Url: https://codereview.webrtc.org/3006173002
Cr-Original-Commit-Position: refs/heads/master@{#19778}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 423227306112c4011a538bf87ae7f2534f39f724
2 files changed