AlternativeServiceServers - Added couple of histograms to measure if

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 58bce2f4386459f28b604adb549978ddf7ca0696..95473c3c3bc4ebc9ededbc31cb74b39975a12625 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -22175,6 +22175,22 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Net.AlternativeServiceServers.MoreCacheEntries">
+  <owner>rtenneti@chromium.org</owner>
+  <summary>
+    The difference between the size of alternative service mappings in cache and
+    preferences.

[Alexei Svitkine (slow), 2015/12/18 19:58:48]

Mention that it's only logged when positive and point to the other histogram.
Same for the other one.

[ramant (doing other things), 2015/12/18 21:34:55]

Added comments. PTAL.

Done.

+  </summary>
+</histogram>
+
+<histogram name="Net.AlternativeServiceServers.MorePrefsEntries">
+  <owner>rtenneti@chromium.org</owner>
+  <summary>
+    The difference between the size of alternative service mappings in
+    preferences and cache.
+  </summary>
+</histogram>
+
 <histogram name="Net.AsyncResourceHandler_PendingDataCount">
   <obsolete>
     Deprecated 03/2015.