Reporting: Add Observer interface to observe cache updates.

net/reporting/reporting_observer.h

+// Copyright 2017 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 NET_REPORTING_REPORTING_OBSERVER_H_
+#define NET_REPORTING_REPORTING_OBSERVER_H_
+
+#include "base/macros.h"
+#include "net/base/net_export.h"
+
+namespace net {
+
+class NET_EXPORT ReportingObserver {
+ public:
+  virtual void OnCacheUpdated();
+
+ protected:
+  ReportingObserver();
+
+  ~ReportingObserver();
+
+  DISALLOW_COPY_AND_ASSIGN(ReportingObserver);
+};
+
+}  // namespace net
+
+#endif  // NET_REPORTING_REPORTING_OBSERVER_H_