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