Add thread annotation macros with RTC_ prefix.
Update samples unittests to use RTC_ versions.

BUG=webrtc:8198

Review-Url: https://codereview.webrtc.org/3011973002
Cr-Original-Commit-Position: refs/heads/master@{#19708}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5301e3c3db996feb4fd622d4449f740da1064477
4 files changed