predictors: Add ResourcePrefetchPredictor database readiness histogram.

chrome/browser/predictors/resource_prefetch_predictor.cc

Index: chrome/browser/predictors/resource_prefetch_predictor.cc
diff --git a/chrome/browser/predictors/resource_prefetch_predictor.cc b/chrome/browser/predictors/resource_prefetch_predictor.cc
index f333d10b6f92ae13d5899553915b79ee16ee6edd..9d729fdf4f84b1dbf6caf69e3d1df77994ac6bf3 100644
--- a/chrome/browser/predictors/resource_prefetch_predictor.cc
+++ b/chrome/browser/predictors/resource_prefetch_predictor.cc
@@ -13,6 +13,7 @@
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/metrics/sparse_histogram.h"
+#include "base/rand_util.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/time/time.h"
 #include "base/trace_event/trace_event.h"
@@ -56,6 +57,8 @@ const char* kFontMimeTypes[] = {"font/woff2",
                                 "application/font-sfnt",
                                 "application/font-ttf"};
 
+const size_t kNumSampleHosts = 50;
+
 // For reporting events of interest that are not tied to any navigation.
 enum ReportingEvent {
   REPORTING_EVENT_ALL_HISTORY_CLEARED = 0,
@@ -609,6 +612,14 @@ void ResourcePrefetchPredictor::OnNavigationComplete(
           base::Bind(&ResourcePrefetchPredictor::OnVisitCountLookup,
                      AsWeakPtr()))),
       &history_lookup_consumer_);
+
+  // Report readiness metric with 20% probability.
+  if (base::RandInt(1, 5) == 5) {
+    history_service->TopHosts(
+        kNumSampleHosts,
+        base::Bind(&ResourcePrefetchPredictor::ReportDatabaseReadiness,
+                   AsWeakPtr()));
+  }
 }
 
 bool ResourcePrefetchPredictor::GetPrefetchData(const GURL& main_frame_url,
@@ -1105,6 +1116,31 @@ void ResourcePrefetchPredictor::LearnRedirect(const std::string& key,
   }
 }
 
+void ResourcePrefetchPredictor::ReportDatabaseReadiness(
+    const history::TopHostsList& top_hosts) {
+  DCHECK_CURRENTLY_ON(BrowserThread::UI);
+  if (top_hosts.size() == 0)
+    return;
+
+  int count_in_cache = 0;
+  for (const std::pair<std::string, int>& top_host : top_hosts) {
+    const std::string& host = top_host.first;
+    if (host_table_cache_->find(host) != host_table_cache_->end()) {
+      ++count_in_cache;
+    } else {
+      // Hostnames in TopHostsLists are stripped of their 'www.' prefix. We
+      // assume that www.foo.com entry from |host_table_cache_| is also suitable
+      // for foo.com.
+      if (!base::StartsWith(host, "www.", base::CompareCase::SENSITIVE) &&
+          host_table_cache_->find("www." + host) != host_table_cache_->end()) {
+        ++count_in_cache;
+      }
+    }
+  }
+  UMA_HISTOGRAM_PERCENTAGE("ResourcePrefetchPredictor.DatabaseReadiness",
+                           100 * count_in_cache / top_hosts.size());
+}
+
 void ResourcePrefetchPredictor::OnURLsDeleted(
     history::HistoryService* history_service,
     bool all_history,