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diff --git a/security/sandbox/chromium/base/synchronization/atomic_flag.h b/security/sandbox/chromium/base/synchronization/atomic_flag.h
new file mode 100644
index 0000000000..f386a16cbd
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+++ b/security/sandbox/chromium/base/synchronization/atomic_flag.h
@@ -0,0 +1,50 @@
+// 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 <stdint.h>
+
+#include <atomic>
+
+#include "base/base_export.h"
+#include "base/macros.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();
+
+  // Set the flag. Must always be called from the same sequence.
+  void 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 {
+    // Inline here: this has a measurable performance impact on base::WeakPtr.
+    return flag_.load(std::memory_order_acquire) != 0;
+  }
+
+  // Resets the flag. Be careful when using this: callers might not expect
+  // IsSet() to return false after returning true once.
+  void UnsafeResetForTesting();
+
+ private:
+  std::atomic<uint_fast8_t> flag_{0};
+  SEQUENCE_CHECKER(set_sequence_checker_);
+
+  DISALLOW_COPY_AND_ASSIGN(AtomicFlag);
+};
+
+}  // namespace base
+
+#endif  // BASE_SYNCHRONIZATION_ATOMIC_FLAG_H_