Update thread annotiation macros to use RTC_ prefix

BUG=webrtc:8198

Review-Url: https://codereview.webrtc.org/3012853002
Cr-Commit-Position: refs/heads/master@{#19760}
diff --git a/webrtc/system_wrappers/source/metrics_default.cc b/webrtc/system_wrappers/source/metrics_default.cc
index f9a6f6a..6e5715e 100644
--- a/webrtc/system_wrappers/source/metrics_default.cc
+++ b/webrtc/system_wrappers/source/metrics_default.cc
@@ -93,7 +93,7 @@
   rtc::CriticalSection crit_;
   const int min_;
   const int max_;
-  SampleInfo info_ GUARDED_BY(crit_);
+  SampleInfo info_ RTC_GUARDED_BY(crit_);
 
   RTC_DISALLOW_COPY_AND_ASSIGN(RtcHistogram);
 };
@@ -165,7 +165,8 @@
 
  private:
   rtc::CriticalSection crit_;
-  std::map<std::string, std::unique_ptr<RtcHistogram>> map_ GUARDED_BY(crit_);
+  std::map<std::string, std::unique_ptr<RtcHistogram>> map_
+      RTC_GUARDED_BY(crit_);
 
   RTC_DISALLOW_COPY_AND_ASSIGN(RtcHistogramMap);
 };