Add new metrics descriptions.

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 9175 matching lines @@
   </summary>
 </histogram>
 
 <histogram name="Platform.CrOSEvent" enum="CrosEventEnum">
   <summary>
     Generic event of interest from Chrome OS.  Intended mainly to help assess
     the frequency of rare error conditions.
   </summary>
 </histogram>
 
+<histogram name="Platform.DiskUsage.Cache_Avg" units="KB">
+  <summary>
+    Average size of user's Cache directory. Logged once a day, if disk usage is
+    high.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.Cache_Max" units="KB">
+  <summary>
+    Maximum size of user's Cache directory. Logged once a day, if disk usage is
+    high.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.Downloads_Avg" units="KB">
+  <summary>
+    Average size of user's Cache directory. Logged once a day, if disk usage is
+    high.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.Downloads_Max" units="KB">
+  <summary>
+    Maximum size of user's Cache directory. Logged once a day, if disk usage is
+    high.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.GCache_Avg" units="KB">
+  <summary>
+    Average size of user's GCache directory. Logged once a day, if disk usage is
+    high.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.GCache_Max" units="KB">
+  <summary>
+    Maximum size of user's GCache directory. Logged once a day, if disk usage is
+    high.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.OldestUserOnDevice">
+  <summary>
+    Days since last login of the least recently user on device. Logged once a
+    day, if disk usage is high.
+  </summary>
+</histogram>
+
+<histogram name="Platform.DiskUsage.UsersOnDevice">
+  <summary>
+    Number of user home dirs on device. Logged once a day, if disk usage is
+    high.
+  </summary>
+</histogram>
+
 <histogram name="Platform.DiskUsageCache" units="KB">
   <summary>KB in use in the /cache filesystem tree. Logged once a day.</summary>
 </histogram>
 
 <histogram name="Platform.DiskUsageChronos" units="KB">
   <summary>
     Chrome OS KB in use in the /home/chronos filesystem tree. Logged once a day
     during log file cleanup.
   </summary>
 </histogram>
@@ skipping 12442 matching lines @@
   <affected-histogram name="Profile.AppCount"/>
 </fieldtrial>
 
 <fieldtrial name="DefaultPinnedApps">
   <group name="Existing"/>
   <group name="Control"/>
   <group name="Alternate"/>
   <affected-histogram name="Cros.ClickOnShelf"/>
 </fieldtrial>
 
+<fieldtrial name="DiskUsagePerUserCount" separator=".">
+  <group name="1User" label="Only 1 user exists on device."/>
+  <group name="2Users" label="2 users exist on device."/>
+  <group name="3Users" label="3 users exist on device."/>
+  <group name="4Users" label="4 users exist on device."/>
+  <group name="5Users" label="5 users exist on device."/>
+  <group name="6Users" label="6 users exist on device."/>
+  <group name="7OrMoreUsers" label="7 or more users exist on device."/>
+  <affected-histogram name="Platform.DiskUsage.Cache_Avg"/>
+  <affected-histogram name="Platform.DiskUsage.Cache_Max"/>
+  <affected-histogram name="Platform.DiskUsage.Downloads_Avg"/>
+  <affected-histogram name="Platform.DiskUsage.Downloads_Max"/>
+  <affected-histogram name="Platform.DiskUsage.GCache_Avg"/>
+  <affected-histogram name="Platform.DiskUsage.GCache_Max"/>
+</fieldtrial>
+
 <fieldtrial name="DnsImpact2">
   <group name="disabled_prefetch"
       label="DNS pre-resolving is disabled in these clients"/>
   <group name="disabled_prefetch_4_connections"
       label="DNS pre-resolving is disabled in these clients, and a maximum of
              4 connections per host was allowed"/>
   <group name="enabled_prefetch_4_connections"
       label="a maximum of 4 connections per host was allowed in these clients"/>
   <group name="parallel_4_prefetch"
       label="DNS pre-resolving was only doing 4 concurrent speculative
@@ skipping 1780 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>