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

base/task_scheduler/task_tracker.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef BASE_TASK_SCHEDULER_TASK_TRACKER_H_
 #define BASE_TASK_SCHEDULER_TASK_TRACKER_H_
 
 #include <memory>
 
 #include "base/base_export.h"
@@ skipping 28 matching lines @@
 
   // Informs this TaskTracker that |task| is about to be posted. Returns true if
   // this operation is allowed (|task| should be posted if-and-only-if it is).
   bool WillPostTask(const Task* task);
 
   // Runs the next Task in |sequence| unless the current shutdown state prevents
   // that. WillPostTask() must have allowed the Task to be posted.
   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).
   bool IsShutdownComplete() const;
 
   // Causes HasShutdownStarted() to return true. Unlike when Shutdown() returns,
   // IsShutdownComplete() won't return true after this returns. Shutdown()
   // cannot be called after this.
   void SetHasShutdownStartedForTesting();
 
@@ skipping 33 matching lines @@
   // Number of BLOCK_SHUTDOWN tasks posted during shutdown.
   HistogramBase::Sample num_block_shutdown_tasks_posted_during_shutdown_ = 0;
 
   DISALLOW_COPY_AND_ASSIGN(TaskTracker);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRACKER_H_