predictors: Add ResourcePrefetchPredictor database readiness histogram.

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 8a133ee1a9f3079e510354276b699b4701a5ce6e..6884b1a77cebfa3d48bb98c8109cb1192fb260b8 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -53228,6 +53228,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.DatabaseReadiness" units="%">
+  <owner>alexilin@chromium.org</owner>
+  <summary>
+    This metric measures the host coverage of the predictor database: On each
+    navigation, it records what percentage of the top X history entries are
+    found in the predictor DB. Caveats: (1) This metric is only recorded once
+    the user has accumulated enough browsing history, implemented as a minimum
+    threshold of total page visits by the user. (2) This metric is sampled, so
+    it's actually only recorded for 1 out of every K navigations.
+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.DbStringTooLong">
   <obsolete>
     Deprecated October 2016. No longer recorded.