Add a mechanism for collecting Rappor samples on a daily interval.

components/metrics/daily_observer.h

Index: components/metrics/daily_observer.h
diff --git a/components/metrics/daily_observer.h b/components/metrics/daily_observer.h
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+++ b/components/metrics/daily_observer.h
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+// 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_DAILY_OBSERVER_H_
+#define COMPONENTS_METRICS_DAILY_OBSERVER_H_
+
+#include "base/macros.h"
+
+namespace metrics {
+
+// DailyObserver receives notifications from RapporService.
+// An observer must be added, removed, and notified on the same thread.
+class DailyObserver {
+public:
+  // Called when daily metrics should be collected.
+  virtual void OnDailyInterval() = 0;
+
+protected:
+  DailyObserver();
+  virtual ~DailyObserver();
+
+private:
+  DISALLOW_COPY_AND_ASSIGN(DailyObserver);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_DAILY_OBSERVER_H_