Reporting: Implement serializer.

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
index fe2ab5c79f1cb4a9780441ed4f72e5a3915d1c1c..86e5c7c77a51e09eb3ac270766329d5a19b20c26 100644
--- a/net/reporting/reporting_observer.h
+++ b/net/reporting/reporting_observer.h
@@ -12,6 +12,24 @@ 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: