Reporting: Add Observer interface to observe cache updates.

net/reporting/reporting_observer.cc

Index: net/reporting/reporting_observer.cc
diff --git a/net/reporting/reporting_observer.cc b/net/reporting/reporting_observer.cc
new file mode 100644
index 0000000000000000000000000000000000000000..5e8d778a61b74a74a70bb6a6ceb8fc6ef7adce7d
--- /dev/null
+++ b/net/reporting/reporting_observer.cc
@@ -0,0 +1,15 @@
+// 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.
+
+#include "net/reporting/reporting_observer.h"
+
+namespace net {
+
+void ReportingObserver::OnCacheUpdated() {}
+
+ReportingObserver::ReportingObserver() {}
+
+ReportingObserver::~ReportingObserver() {}
+
+}  // namespace net