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

components/metrics/daily_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 COMPONENTS_METRICS_DAILY_OBSERVER_H_
+#define COMPONENTS_METRICS_DAILY_OBSERVER_H_
+
+#include "base/macros.h"
+
+namespace metrics {
+
+// DailyObserver receives notifications from a DailyInterval.

[Alexei Svitkine (slow), 2014/09/15 17:50:38]

For an interface this simple, I suggest defining it inline in daily_interval.h,
similar to what we do here:

https://code.google.com/p/chromium/codesearch#chromium/src/base/metrics/field...

(Then it will be DailyInterval::Observer).

[Steven Holte, 2014/09/15 20:17:10]

Done.

+// Observers must be added before the DailyInterval begins checking time,
+// and will be owned by the DailyInterval.
+class DailyObserver {
+ public:
+  DailyObserver();
+  virtual ~DailyObserver();
+
+  // Called when daily metrics should be collected.
+  virtual void OnDailyInterval() = 0;
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(DailyObserver);
+};
+
+}  // namespace metrics
+
+#endif  // COMPONENTS_METRICS_DAILY_OBSERVER_H_