predictors: Add prefetcher 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 778bde8307e04d91858887fe90fe838ec55fbd2d..3fa7f73cade15eb917bae51aab1d1792dca99e09 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -54077,6 +54077,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.CachePattern"
+    enum="HttpCachePattern">
+  <owner>alexilin@chromium.org</owner>
+  <summary>
+    Like HttpCache.Pattern but only for requests made during speculative
+    prefetch. Logged per request.

[Benoit L, 2017/01/25 17:22:34]

nit: Logged once per successful request.

[alexilin, 2017/01/25 17:47:33]

Right. Done.

+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.DatabaseReadiness" units="%">
   <owner>alexilin@chromium.org</owner>
   <summary>
@@ -54506,6 +54515,21 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ResourcePrefetchPredictor.PrefetchedCount">
+  <owner>alexilin@chromium.org</owner>
+  <summary>
+    The count of successfully prefetched subresources for a single page.

[Benoit L, 2017/01/25 17:22:35]

nit: Single prefetch request?

[alexilin, 2017/01/25 17:47:33]

Won't it be confused with URL request that prefetcher initiates? 
I think it would be better to stick to a prefetcher instance either a page.

+  </summary>
+</histogram>
+
+<histogram name="ResourcePrefetchPredictor.PrefetchedSizeKB" units="KB">
+  <owner>alexilin@chromium.org</owner>
+  <summary>
+    The total amount of data consumed by predictor for a single page. Only
+    successfully finished requests taken into account.
+  </summary>
+</histogram>
+
 <histogram name="ResourcePrefetchPredictor.ReportingEvent"
     enum="ResourcePrefetchPredictorReportingEvent">
   <owner>zhenw@chromium.org</owner>