Update thread annotiation macros to use RTC_ prefix

BUG=webrtc:8198

Review-Url: https://codereview.webrtc.org/3012853002
Cr-Original-Commit-Position: refs/heads/master@{#19760}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a37de39216cccecb1448725b7dbd00bb69c52360