predictors: Add ResourcePrefetchPredictor database readiness histogram.

chrome/browser/predictors/resource_prefetch_predictor.h

 // Copyright 2014 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 CHROME_BROWSER_PREDICTORS_RESOURCE_PREFETCH_PREDICTOR_H_
 #define CHROME_BROWSER_PREDICTORS_RESOURCE_PREFETCH_PREDICTOR_H_
 
 #include <stddef.h>
 
 #include <map>
@@ skipping 293 matching lines @@
                        RedirectDataMap* redirect_map);
 
   // Updates information about final redirect destination for |key| in
   // |redirect_map| and correspondingly updates the predictor database.
   void LearnRedirect(const std::string& key,
                      PrefetchKeyType key_type,
                      const std::string& final_redirect,
                      size_t max_redirect_map_size,
                      RedirectDataMap* redirect_map);
 
-  // Reports overall page load time.

[alexilin, 2017/01/11 10:06:16]

Oops! These are undefined and unused methods.

-  void ReportPageLoadTimeStats(base::TimeDelta plt) const;
-
-  // Reports page load time for prefetched and not prefetched pages.
-  void ReportPageLoadTimePrefetchStats(
-      base::TimeDelta plt,
-      bool prefetched,
-      base::Callback<void(int)> report_network_type_callback,
-      PrefetchKeyType key_type) const;
+  // Reports database readiness metric defined as percentage of navigated hosts
+  // found in DB for last X entries in history.
+  void ReportDatabaseReadiness(const history::TopHostsList& top_hosts);
 
   // history::HistoryServiceObserver:
   void OnURLsDeleted(history::HistoryService* history_service,
                      bool all_history,
                      bool expired,
                      const history::URLRows& deleted_rows,
                      const std::set<GURL>& favicon_urls) override;
   void OnHistoryServiceLoaded(
       history::HistoryService* history_service) override;
 
@@ skipping 50 matching lines @@
 
  private:
   ResourcePrefetchPredictor* predictor_;
 
   DISALLOW_COPY_AND_ASSIGN(TestObserver);
 };
 
 }  // namespace predictors
 
 #endif  // CHROME_BROWSER_PREDICTORS_RESOURCE_PREFETCH_PREDICTOR_H_