Add AtomicFlag class to base/......

base/atomic_flag.cc

+// Copyright (c) 2009 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.
+
+#include "base/atomic_flag.h"
+
+#include "base/dynamic_annotations.h"
+#include "base/logging.h"
+
+namespace base {
+
+void AtomicFlag::Set() {
+  DCHECK(!flag_);
+  // Set() method can't be called if the flag has already been set.
+
+  ANNOTATE_HAPPENS_BEFORE(&flag_);
+  base::subtle::Release_Store(&flag_, 1);
+}
+
+bool AtomicFlag::IsSet() const {
+  bool ret = base::subtle::Acquire_Load(&flag_) != 0;
+  if (ret) {
+    ANNOTATE_HAPPENS_AFTER(&flag_);
+  }
+  return ret;
+}
+
+}  // namespace base