Add prefetch UMA metrics.

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 428012cccd793369c938a493a5ee9c6ed360926b..b7a6f3ea423f52e558f251d72241c7107d1a9a9e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -31301,6 +31301,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Precache.Fetch.PercentCompleted" units="percentage">
+  <owner>twifkak@chromium.org</owner>
+  <summary>
+    The percent of manifests for which all respective resources have been

[Mark P, 2015/06/26 17:03:37]

respective?
I don't know what this word means in the context of this sentence.
If it means something, you need to explain it or choose a better word.
Otherwise remove it.

[twifkak, 2015/06/26 17:48:57]

Done.

+    downloaded. Logged per prefetch run.
+  </summary>
+</histogram>
+
+<histogram name="Precache.Fetch.ResponseBytes" units="bytes">
+  <owner>twifkak@chromium.org</owner>
+  <summary>
+    The total number of response bytes received from all prefetch requests,
+    including config, manifests, and resources. Logged per prefetch run.
+  </summary>
+</histogram>
+
 <histogram name="Precache.Saved" units="bytes">
   <owner>bengr@chromium.org</owner>
   <summary>