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}

rtc_base/sequenced_task_checker.h

@@ -22,6 +22,12 @@
 #include "rtc_base/thread_annotations.h"
 
 namespace rtc {
+namespace internal {
+// Forward declaration of the internal implementation of RTC_GUARDED_BY().
+// SequencedTaskChecker grants this class access to call its IsCurrent() method.
+// See thread_checker.h for more details.
+class AnnounceOnThread;
+}  // namespace internal
 
 // Do nothing implementation, for use in release mode.
 //
@@ -30,8 +36,11 @@ namespace rtc {
 class SequencedTaskCheckerDoNothing {
  public:
   bool CalledSequentially() const { return true; }
-
   void Detach() {}
+
+ private:
+  friend class internal::AnnounceOnThread;
+  bool IsCurrent() const { return CalledSequentially(); }
 };
 
 // SequencedTaskChecker is a helper class used to help verify that some methods

rtc_base/sequenced_task_checker_impl.h

@@ -34,6 +34,9 @@ class SequencedTaskCheckerImpl {
   void Detach();
 
  private:
+  friend class internal::AnnounceOnThread;
+  bool IsCurrent() const { return CalledSequentially(); }
+
   typedef const void* QueueId;
   CriticalSection lock_;
   ThreadChecker thread_checker_;

rtc_base/sequenced_task_checker_unittest.cc

@@ -9,15 +9,38 @@
  */
 
 #include "rtc_base/sequenced_task_checker.h"
+
 #include "rtc_base/checks.h"
 #include "rtc_base/constructormagic.h"
 #include "rtc_base/platform_thread.h"
 #include "rtc_base/task_queue.h"
+#include "rtc_base/thread_checker.h"
 #include "test/gtest.h"
 
 namespace rtc {
 
 namespace {
+
+// This class is dead code, but its purpose is to make sure that
+// SequencedTaskChecker is compatible with the RTC_GUARDED_BY and RTC_RUN_ON
+// attributes that are checked at compile-time.
+class CompileTimeTestForGuardedBy {
+ public:
+  int CalledOnSequence() RTC_RUN_ON(sequence_checker_) {
+    return guarded_;
+  }
+
+  void CallMeFromSequence() {
+    RTC_DCHECK_RUN_ON(&sequence_checker_)
+        << "Should be called on sequence";
+  }
+
+ private:
+  int guarded_ RTC_GUARDED_BY(sequence_checker_);
+  rtc::SequencedTaskChecker sequence_checker_;
+};
+
+
 // Calls SequencedTaskChecker::CalledSequentially on another thread.
 class CallCalledSequentiallyOnThread {
  public: