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 during ReportingContext::Initialize, before any calls to
+  // OnContextInitialized.
+  //
+  // This is the correct time to preload or restore data in the
+  // ReportingCache, since other observers won't act on it until all observers
+  // have had a chance to initialize.
+  virtual void OnContextInitializing();
+
+  // Called during ReportingContext::Initialize, after all calls to
+  // OnContextInitializing.
+  //
+  // This is the correct time to start timers or network requests based on data
+  // restored by other observers.
+  virtual void OnContextInitialized();
+
+  // Once ReportingContext::Initialize has called OnContextInitializing on all
+  // Observers, OnCacheUpdated will be called on any update to the
+  // ReportingCache.
   virtual void OnCacheUpdated();
 
  protected:
   ReportingObserver();
 
   ~ReportingObserver();
 
   DISALLOW_COPY_AND_ASSIGN(ReportingObserver);
 };
 
 }  // namespace net
 
 #endif  // NET_REPORTING_REPORTING_OBSERVER_H_