Loosen/improve AtomicFlag semantics.

base/synchronization/atomic_flag.h

 // Copyright (c) 2011 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_SYNCHRONIZATION_ATOMIC_FLAG_H_
 #define BASE_SYNCHRONIZATION_ATOMIC_FLAG_H_
 
 #include "base/atomicops.h"
 #include "base/base_export.h"
 #include "base/macros.h"
-#include "base/threading/thread_checker.h"
+#include "base/sequence_checker.h"
 
 namespace base {
 
 // A flag that can safely be set from one thread and read from other threads.
 //
 // This class IS NOT intended for synchronization between threads.
 class BASE_EXPORT AtomicFlag {
  public:
   AtomicFlag();
   ~AtomicFlag() = default;
 
-  // Set the flag. May only be called on the thread which created the object.
+  // Set the flag. Must always be called from the same sequence.
   void Set();
 
-  // Returns true iff the flag was set.
+  // Returns true iff the flag was set. If this returns true, the current thread
+  // is guaranteed to be synchronized with all memory operations on the sequence
+  // which invoked Set() up until at least the first call to Set() on it.
   bool IsSet() const;
 
   // Resets the flag. Be careful when using this: callers might not expect
   // IsSet() to return false after returning true once.
   void UnsafeResetForTesting();
 
  private:
   base::subtle::Atomic32 flag_ = 0;
-  ThreadChecker thread_checker_;
+  SequenceChecker set_sequence_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(AtomicFlag);
 };
 
 }  // namespace base
 
 #endif  // BASE_SYNCHRONIZATION_ATOMIC_FLAG_H_