Update thread annotiation macros to use RTC_ prefix

webrtc/system_wrappers/source/metrics_default.cc

Index: webrtc/system_wrappers/source/metrics_default.cc
diff --git a/webrtc/system_wrappers/source/metrics_default.cc b/webrtc/system_wrappers/source/metrics_default.cc
index f9a6f6aaf906cb8a697fed920cc675635a7e9405..6e5715ea232f8bfcb289f602ebe0c31aed36f9a4 100644
--- a/webrtc/system_wrappers/source/metrics_default.cc
+++ b/webrtc/system_wrappers/source/metrics_default.cc
@@ -93,7 +93,7 @@ class RtcHistogram {
   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 @@ class RtcHistogramMap {
 
  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);
 };