Reporting: Add Observer interface to observe cache updates.

net/reporting/reporting_observer.h

Index: net/reporting/reporting_observer.h
diff --git a/net/reporting/reporting_observer.h b/net/reporting/reporting_observer.h
new file mode 100644
index 0000000000000000000000000000000000000000..fe2ab5c79f1cb4a9780441ed4f72e5a3915d1c1c
--- /dev/null
+++ b/net/reporting/reporting_observer.h
@@ -0,0 +1,27 @@
+// 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_