predictors: Add subresources count and prefetching duration histograms.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index eef0a376034b7a9d51553b265a49c94b159950ff..dc57e0d6e7e98f4cfa27c3bed775a6fdaf0169d1 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -54326,6 +54326,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.LearningCount" units="urls">
+  <owner>alexilin@chromium.org</owner>
+  <summary>
+    When the prefetch predictor has resources in the local database for a given
+    navigation, the count of predicted urls.
+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.LearningPrecision" units="%">
   <owner>lizeb@chromium.org</owner>
   <summary>
@@ -54490,6 +54498,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.PrefetchingDuration" units="ms">
+  <owner>alexilin@chromium.org</owner>
+  <summary>
+    Amount of time available for prefetching. Specifically, this is a time
+    interval between corresponding StartPrefetching() and StopPrefetching()
+    calls. This is recorded for both prefetched and non-prefetched pages.
+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.ReportingEvent"
     enum="ResourcePrefetchPredictorReportingEvent">
   <owner>alexilin@chromium.org</owner>