Split out MetricsServiceAccessor out of ChromeMetricsServiceAccessor.

chrome/browser/metrics/metrics_service_accessor.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 CHROME_BROWSER_METRICS_METRICS_SERVICE_ACCESSOR_H_
+#define CHROME_BROWSER_METRICS_METRICS_SERVICE_ACCESSOR_H_
+
+#include "base/macros.h"
+
+class MetricsServiceObserver;
+
+// This class limits and documents access to metrics service helper methods.
+// These methods are protected so each user has to inherit own program-specific
+// specialization and enable access there by declaring friends.
+class MetricsServiceAccessor {
+ protected:
+  // Constructor declared as protected to enable inheritance. Decendants have to

[Ilya Sherman, 2014/05/21 13:18:30]

nit: "have to" -> "should"

[motek., 2014/05/21 14:41:09]

Done.

+  // disallow instantiation.
+  MetricsServiceAccessor() {}
+
+  // Registers/unregisters |observer| to receive MetricsLog notifications
+  // from metrics service.
+  static void AddMetricsServiceObserver(MetricsServiceObserver* observer);
+  static void RemoveMetricsServiceObserver(MetricsServiceObserver* observer);
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MetricsServiceAccessor);
+};
+
+#endif  // CHROME_BROWSER_METRICS_METRICS_SERVICE_ACCESSOR_H_