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 59310e105767ca3eebf87f350b0b988231a54dbd..4320b49067f4e212ca2beb5dea724ab85baa6a5a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -12895,6 +12895,15 @@ 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

[Mark P, 2015/06/12 20:08:59]

What happens if hosts get reordered?  I visit host #5 several times and then it
becomes host #4?

[twifkak, 2015/06/12 20:43:46]

Done.

+    into the 51st bucket. Only count the page visit if it came from user
+    browsing and only count it once when cycling through a redirect chain.

[Mark P, 2015/06/12 20:08:59]

"if it came from user browsing"
as opposed to what?

[twifkak, 2015/06/12 20:43:46]

I'm not sure to what it's referring exactly; I just copied it from the other
histogram description (since it's a mirror of that CL). I'd guess it's referring
to this: "visit_source == SOURCE_BROWSED && (transition &
ui::PAGE_TRANSITION_CHAIN_END)", in which case, it would be as opposed to the
other things on those enums:

https://code.google.com/p/chromium/codesearch#chromium/src/components/history...
https://code.google.com/p/chromium/codesearch#chromium/src/ui/base/page_trans...

e.g. redirects, automated sync, extensions, etc.\

+  </summary>
+</histogram>
+
 <histogram name="History.TopSitesRecoveredPercentage" units="%">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>
@@ -12923,18 +12932,6 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
-<histogram name="History.TopSitesVisitsByRank" units="rank">

[Mark P, 2015/06/12 20:08:59]

Do not delete old histograms.

[twifkak, 2015/06/12 20:43:46]

Done.

-  <obsolete>
-    Deprecated as of 11/2014.
-  </obsolete>
-  <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
-  <summary>
-    Page visits to each of a user's top 50 sites. Visits to all other sites go
-    into the 51st bucket. Only count the page visit if it came from user
-    browsing and only count it once when cycling through a redirect chain.
-  </summary>
-</histogram>
-
 <histogram name="History.UpdateTopSitesOnDBThreadTime" units="milliseconds">
   <obsolete>
     Deprecated because only the execution time at startup is of interest. See