Add Extensions.BackgroundPageType histogram.

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 b8f631645580ba5bb8014c75829994d4c99806bf..49a20f344821fc9bfffa391f6ea8024faf825dfd 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1654,6 +1654,11 @@ other types of suffix sets.
   </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>
@@ -11625,6 +11630,12 @@ other types of suffix sets.
   <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"/>