predictors: Add precision / recall histograms for speculative prefetch.

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 78a0d5c2e7dc58675eccfbc51a9fbf4bfedf9dcd..9e04a2c3c7554529e13316845505c3442081e74e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -53961,6 +53961,24 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.LearningPrecision" units="%">
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    When the prefetch predictor has resources in the local database for a given
+    navigation, the precision of the predictions, in percentage. This is
+    computed as 100 \times \frac{correct predictions}{predictions}.
+  </summary>
+</histogram>
+
+<histogram name="ResourcePrefetchPredictor.LearningRecall" units="%">
+  <owner>lizeb@chromium.org</owner>
+  <summary>
+    When the prefetch predictor has resources in the local database for a given
+    navigation, the recall of the predictions, in percentage. This is computed
+    as 100 \times \frac{correct predictions}{all prefetchable subresources}.
+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.MainFrameRequestStats"
     enum="ResourcePrefetchPredictorMainFrameRequestStats">
   <owner>zhenw@chromium.org</owner>