To reduce usage of rtc::MessageHandler, hide rtc::Thread::Send into private section with intention to deprecate it in favor of the new Invoke function. Bug: webrtc:9702, webrtc:11318 Change-Id: Ib4c26f9abc361e05a45b2a91929af58ab160b3f0 Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/274166 Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org> Commit-Queue: Danil Chapovalov <danilchap@webrtc.org> Cr-Commit-Position: refs/heads/main@{#38036}