Since rtc::Thread is the only class inheriting from rtc::MessageQueue and most members of MessageQueue are public or protected the split is not adding much value. In preparation for future cleanup, this cl merges the two classes. Bug: webrtc:9883 Change-Id: Ia0efb4349f66f653aa34fa4d244998f187e3ce36 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/165340 Commit-Queue: Sebastian Jansson <srte@webrtc.org> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org> Reviewed-by: Steve Anton <steveanton@webrtc.org> Cr-Commit-Position: refs/heads/master@{#30235}