Extract the handling of the clean exit beacon from MetricsService. The extracted implementation sup…

components/metrics/clean_exit_beacon.h

+// Copyright 2014 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 COMPONENTS_METRICS_CLEAN_EXIT_BEACON_H_
+#define COMPONENTS_METRICS_CLEAN_EXIT_BEACON_H_
+
+#include "base/macros.h"
+#include "base/strings/string16.h"
+
+class PrefService;
+
+namespace metrics {
+
+// Reads and updates a beacon used to detect whether the previous browser
+// process exited cleanly.
+class CleanExitBeacon {
+ public:
+  // Instantiates a CleanExitBeacon whose value is stored in |local_state|.
+  // |local_state| must be fully initialized.
+  // On Windows, |backup_registry_key| is used to store a backup of the beacon.
+  // It is ignored on other platforms.
+  CleanExitBeacon(
+      const base::string16& backup_registry_key,
+      PrefService* local_state);
+
+  ~CleanExitBeacon();
+
+  // Returns the original value of the beacon.
+  bool exited_cleanly() const { return initial_value_; }
+
+  // Writes the provided beacon value.
+  void WriteBeaconValue(bool exited_cleanly);
+
+ private:
+  PrefService* const local_state_;
+  const bool initial_value_;
+  const base::string16 backup_registry_key_;
+
+  DISALLOW_COPY_AND_ASSIGN(CleanExitBeacon);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_CLEAN_EXIT_BEACON_H_