Extend expiration of Extensions.PowerfulCount

I consider this histogram relevant for the foreseeable future, thus
extending by 1 year.

Bug: 1140039
Change-Id: I00c25bb7ddb39ca2ad7b3ef441da635396cc8e6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2526343
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Auto-Submit: Thiemo Nagel <tnagel@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825351}

tools/metrics/histograms/histograms_xml/extensions/histograms.xml

@@ -2929,13 +2929,13 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Extensions.PowerfulCount" units="extensions"
-    expires_after="2020-12-01">
+    expires_after="2021-12-01">
   <owner>tnagel@chromium.org</owner>
   <owner>chrome-privacy-core@chromium.org</owner>
   <summary>
-    The number of extensions reported in chrome://management ui with powerful
+    The number of extensions reported in chrome://management UI with powerful
     permissions. This number is recorded every time the user opens
-    chrome://management ui.
+    chrome://management UI.
   </summary>
 </histogram>