Update thread annotiation macros to use RTC_ prefix

webrtc/media/base/adaptedvideotracksource.h

 /*
  *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 56 matching lines @@
   // Part of VideoTrackSourceInterface.
   bool GetStats(Stats* stats) override;
 
   void OnSinkWantsChanged(const rtc::VideoSinkWants& wants);
 
   rtc::ThreadChecker thread_checker_;
 
   cricket::VideoAdapter video_adapter_;
 
   rtc::CriticalSection stats_crit_;
-  rtc::Optional<Stats> stats_ GUARDED_BY(stats_crit_);
+  rtc::Optional<Stats> stats_ RTC_GUARDED_BY(stats_crit_);
 
   VideoBroadcaster broadcaster_;
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_MEDIA_BASE_ADAPTEDVIDEOTRACKSOURCE_H_