Rename thread annotation macros to have RTC prefix for syncrhonization primitives.

other macros (e.g. GUARDED_BY) rename postpone to followup CL
since it touches codebase wider

BUG=webrtc:8198

Review-Url: https://codereview.webrtc.org/3004393002
Cr-Commit-Position: refs/heads/master@{#19701}
diff --git a/webrtc/rtc_base/task_queue.h b/webrtc/rtc_base/task_queue.h
index 5dee6e6..08cde56 100644
--- a/webrtc/rtc_base/task_queue.h
+++ b/webrtc/rtc_base/task_queue.h
@@ -153,7 +153,7 @@
 // TaskQueue itself has been deleted or it may happen synchronously while the
 // TaskQueue instance is being deleted.  This may vary from one OS to the next
 // so assumptions about lifetimes of pending tasks should not be made.
-class LOCKABLE TaskQueue {
+class RTC_LOCKABLE TaskQueue {
  public:
   // TaskQueue priority levels. On some platforms these will map to thread
   // priorities, on others such as Mac and iOS, GCD queue priorities.