Update thread annotiation macros in modules to use RTC_ prefix

BUG=webrtc:8198

Review-Url: https://codereview.webrtc.org/3010223002
Cr-Commit-Position: refs/heads/master@{#19728}
diff --git a/webrtc/modules/video_coding/generic_decoder.h b/webrtc/modules/video_coding/generic_decoder.h
index fcfea77..c8122da 100644
--- a/webrtc/modules/video_coding/generic_decoder.h
+++ b/webrtc/modules/video_coding/generic_decoder.h
@@ -69,7 +69,7 @@
   VCMReceiveCallback* _receiveCallback = nullptr;
   VCMTiming* _timing;
   rtc::CriticalSection lock_;
-  VCMTimestampMap _timestampMap GUARDED_BY(lock_);
+  VCMTimestampMap _timestampMap RTC_GUARDED_BY(lock_);
   uint64_t _lastReceivedPictureID;
   int64_t ntp_offset_;
 };