Update thread annotiation macros in rtc_base to use RTC_ prefix

BUG=webrtc:8198

Review-Url: https://codereview.webrtc.org/3006133002
Cr-Original-Commit-Position: refs/heads/master@{#19714}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3c6abd200cb9865402c96466fafe6867730cb2be