Add Extensions.BackgroundPageType histogram.

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 1636 matching lines @@
     extension_misc::AppLaunchBuckets. See also Apps.AppLaunch for v2 apps.
   </summary>
 </histogram>
 
 <histogram name="Extensions.AppTabLaunchType" enum="ExtensionLaunchType">
   <summary>
     The number of apps launched grouped by ExtensionPrefs::LaunchType.
   </summary>
 </histogram>
 
+<histogram name="Extensions.BackgroundPageType"
+    units="ExtensionBackgroundPageType">
+  <summary>The type (if any) of background page the extension has.</summary>

[jar (doing other things), 2013/06/04 23:53:59]

nit: Please indicate when this is samples, such as:

This is recorded when an extension is installed(??).

[Matt Perry, 2013/06/05 00:04:22]

Done.

+</histogram>
+
 <histogram name="Extensions.DisabledUIUserResponse"
     enum="ExtensionDisabledUIUserResponse">
   <summary>
     User response to the dialog shown when an extension is disabled due to an
     update requiring more permissions.
   </summary>
 </histogram>
 
 <histogram name="Extensions.ExtensionInstalled">
   <summary>An extension has been installed.</summary>
@@ skipping 9951 matching lines @@
   <int value="10044" label="WSAESOCKTNOSUPPORT"/>
   <int value="10047" label="WSAEAFNOSUPPORT"/>
   <int value="10093" label="WSANOTINITIALISED"/>
   <int value="10109" label="WSA_TYPE_NOT_FOUND"/>
   <int value="11001" label="WSA_HOST_NOT_FOUND"/>
   <int value="11002" label="WSATRY_AGAIN"/>
   <int value="11003" label="WSA_ANO_RECOVERY"/>
   <int value="11004" label="WSANO_DATA"/>
 </enum>
 
+<enum name="ExtensionBackgroundPageType" type="int">
+  <int value="0" label="None"/>
+  <int value="1" label="Persistent"/>
+  <int value="2" label="Event Page"/>
+</enum>
+
 <enum name="ExtensionDisabledUIUserResponse" type="int">
   <int value="0" label="IGNORED"/>
   <int value="1" label="REENABLE"/>
   <int value="2" label="UNINSTALL"/>
 </enum>
 
 <enum name="ExtensionFunctions" type="int">
 <!-- Generated from ../../../chrome/browser/extensions/extension_function_histogram_value.h -->
 
   <int value="0" label="UNKNOWN"/>
@@ skipping 5341 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>