Log operator code histogram on new metric log

chrome/browser/metrics/metrics_service_observer.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_OBSERVER_H_
+#define CHROME_BROWSER_METRICS_METRICS_SERVICE_OBSERVER_H_
+
+#include "base/macros.h"
+
+// MetricsServiceObserver receives notifications from MetricsService.
+// An observer must be added, removed, and notified on the same thread.
+class MetricsServiceObserver {
+ public:
+  // Called when a new MetricsLog is created.
+  virtual void OnDidCreateMetricsLog() = 0;
+
+ protected:
+  MetricsServiceObserver();
+  virtual ~MetricsServiceObserver();
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(MetricsServiceObserver);
+};
+
+#endif  // CHROME_BROWSER_METRICS_METRICS_SERVICE_OBSERVER_H_