Update expiry for Webapp.Update.ManifestUpdateResult histogram

This histogram is still useful for monitoring web app manifest updating
issues.

Bug: 1153779
Change-Id: Ib8cdc18d88755c51574abb109cd039ee81aa77c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2592168
Auto-Submit: Alan Cutter <alancutter@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840864}

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -17599,8 +17599,9 @@ regressions. -->
 </histogram>
 
 <histogram name="Webapp.Update.ManifestUpdateResult"
-    enum="WebAppManifestUpdateResult" expires_after="M89">
+    enum="WebAppManifestUpdateResult" expires_after="M92">
   <owner>alancutter@chromium.org</owner>
+  <owner>desktop-pwas-team@chromium.org</owner>
   <owner>loyso@chromium.org</owner>
   <owner>raymes@chromium.org</owner>
   <summary>