Update thread annotiation macros in rtc_base to use RTC_ prefix

BUG=webrtc:8198

Review-Url: https://codereview.webrtc.org/3006133002
Cr-Commit-Position: refs/heads/master@{#19714}

webrtc/rtc_base/bufferqueue.h

diff --git a/webrtc/rtc_base/bufferqueue.h b/webrtc/rtc_base/bufferqueue.h
index 3abb8cf..82f1ac1 100644
--- a/webrtc/rtc_base/bufferqueue.h
+++ b/webrtc/rtc_base/bufferqueue.h
@@ -50,8 +50,8 @@
   size_t capacity_;
   size_t default_size_;
   CriticalSection crit_;
-  std::deque<Buffer*> queue_ GUARDED_BY(crit_);
-  std::vector<Buffer*> free_list_ GUARDED_BY(crit_);
+  std::deque<Buffer*> queue_ RTC_GUARDED_BY(crit_);
+  std::vector<Buffer*> free_list_ RTC_GUARDED_BY(crit_);
 
   RTC_DISALLOW_COPY_AND_ASSIGN(BufferQueue);
 };