Track fraction of visits to top URLs.

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 3117 matching lines @@
   <summary>
     Number of total files (regualr files + hosted documents) on Drive.  Logged
     when Drive is first accessed.
   </summary>
 </histogram>
 
 <histogram name="GoogleNow.Event" enum="GoogleNowEvent">
   <summary>Events in Google Now component extension.</summary>
 </histogram>
 
+<histogram name="History.TopSitesVisitsByRank" units="rank">
+  <summary>
+    Page visits to each of a user's top 50 sites. Visits to all other sites go
+    into the 51st bucket.
+  </summary>
+</histogram>
+
 <histogram name="HistoryPage.OtherSessionsMenu" enum="NtpOtherSessionsType">
   <summary>
     Histogram for usage of the section in the history page that allows the user
     to access tabs from other devices.
   </summary>
 </histogram>
 
 <histogram name="HttpCache.EntryLockWait" units="milliseconds">
   <summary>
     The time spent waiting for write lock on a disk cache entry.
@@ skipping 16474 matching lines @@
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>
   <group name="PlusIcon" label="Plus icon in apps page"/>
   <affected-histogram name="Extensions.AppLaunch"/>
   <affected-histogram name="NewTabPage.DefaultPageType"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>