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/video_coding_impl.cc b/webrtc/modules/video_coding/video_coding_impl.cc
index 7ab5314..7551214 100644
--- a/webrtc/modules/video_coding/video_coding_impl.cc
+++ b/webrtc/modules/video_coding/video_coding_impl.cc
@@ -72,7 +72,7 @@
 
  private:
   rtc::CriticalSection cs_;
-  EncodedImageCallback* callback_ GUARDED_BY(cs_);
+  EncodedImageCallback* callback_ RTC_GUARDED_BY(cs_);
 };
 
 class VideoCodingModuleImpl : public VideoCodingModule {