Add support for RTC_GUARDED_BY to SequencedTaskChecker.

Bug: webrtc:8903
Change-Id: I5121ac8412fd60694ea9b4abf0984bc825c1aa18
Reviewed-on: https://webrtc-review.googlesource.com/54311
Commit-Queue: Tommi <tommi@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22079}
diff --git a/rtc_base/sequenced_task_checker_impl.h b/rtc_base/sequenced_task_checker_impl.h
index 0a5464e..86d5ef0 100644
--- a/rtc_base/sequenced_task_checker_impl.h
+++ b/rtc_base/sequenced_task_checker_impl.h
@@ -34,6 +34,9 @@
   void Detach();
 
  private:
+  friend class internal::AnnounceOnThread;
+  bool IsCurrent() const { return CalledSequentially(); }
+
   typedef const void* QueueId;
   CriticalSection lock_;
   ThreadChecker thread_checker_;