Fix incorrect memory barrier usage I previously asked to be introduced.

base/task_scheduler/task_tracker.h

Index: base/task_scheduler/task_tracker.h
diff --git a/base/task_scheduler/task_tracker.h b/base/task_scheduler/task_tracker.h
index 80cc3c02870685ac421f4bb8f4278c376355e091..44309e503eb55c5f08361447748a687981f77cf8 100644
--- a/base/task_scheduler/task_tracker.h
+++ b/base/task_scheduler/task_tracker.h
@@ -46,7 +46,9 @@ class BASE_EXPORT TaskTracker {
   void RunNextTaskInSequence(const Sequence* sequence);
 
   // Returns true once shutdown has started (Shutdown() has been called but
-  // might not have returned).
+  // might not have returned). Note: sequential consistency with the thread
+  // calling Shutdown() (or SetHasShutdownStartedForTesting()) isn't guaranteed
+  // by this call.
   bool HasShutdownStarted() const;
 
   // Returns true if shutdown has completed (Shutdown() has returned).