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..65479dbc63e28d830a52feb15cfba61a1a32c8f5 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1654,6 +1654,14 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Extensions.BackgroundPageType"
+    units="ExtensionBackgroundPageType">
+  <summary>
+    The type (if any) of background page the extension has. Recorded for
+    installed extensions on startup.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.DisabledUIUserResponse"
     enum="ExtensionDisabledUIUserResponse">
   <summary>
@@ -11625,6 +11633,12 @@ 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"/>