predictors: Add ResourcePrefetchPredictor database readiness histogram.

chrome/browser/predictors/resource_prefetch_predictor.h

Index: chrome/browser/predictors/resource_prefetch_predictor.h
diff --git a/chrome/browser/predictors/resource_prefetch_predictor.h b/chrome/browser/predictors/resource_prefetch_predictor.h
index ae2a57c72d127ebb588d5682bd5ad5ab81bdbc3e..ecadc004c1f2f49c644b2f834ab58f71d708ab6f 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor.h
+++ b/chrome/browser/predictors/resource_prefetch_predictor.h
@@ -311,15 +311,9 @@ class ResourcePrefetchPredictor
                      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,