Add History.TopHostsVisitsByRank UMA metric.

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 9af6b4ad2287957aa5895db90a742ed8393e6c4b..f08e1bd55b37a9963d463cca39ced350f1144024 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -13079,6 +13079,18 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="History.TopHostsVisitsByRank" units="rank">
+  <owner>twifkak@chromium.org</owner>
+  <summary>
+    Page visits to each of a user's top 50 hosts. Visits to all other hosts go
+    into the 51st bucket. Android-only. Only counts the last URL in a redirect

[sky, 2015/07/01 16:53:33]

What makes this android only? Also, document that it's 1s based.

[twifkak, 2015/07/01 19:58:40]

The host_ranks_ map is updated only by TopHosts(), which is called only by
precache::PrecacheManager::StartPrecaching(), which is called only by
PrecacheLauncher::Start(), which is a JNI class.
Done.

+    chain. The list of top hosts is computed approximately nightly. This means
+    that a given day will likely include a recalculation of top hosts, and
+    therefore a given bucket may represent two different hosts at different
+    times of day.
+  </summary>
+</histogram>
+
 <histogram name="History.TopSitesRecoveredPercentage" units="%">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>