Reporting: Implement serializer.

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:
+  // Called whenever any change is made to the ReportingCache.
   virtual void OnCacheUpdated();
 
  protected:
   ReportingObserver();
 
   ~ReportingObserver();
 
   DISALLOW_COPY_AND_ASSIGN(ReportingObserver);
 };
 
 }  // namespace net
 
 #endif  // NET_REPORTING_REPORTING_OBSERVER_H_