Extend expiry of ChromeOS.Settings load time metrics

OS settings are being migrated from Polymer 2 to Polymer 3, so load time
performance metrics are still valuable. Extend both load time metrics
to June 2021.

Bug: 1121211
Change-Id: I9b7b59f52edd8bb580f662a33470475e605349bd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436294
Commit-Queue: James Cook <jamescook@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: James Cook <jamescook@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811366}

tools/metrics/histograms/histograms_xml/chrome/histograms.xml

@@ -904,7 +904,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="ChromeOS.Settings.LoadCompletedTime" units="ms"
-    expires_after="2020-10-04">
+    expires_after="2021-06-01">
   <owner>jamescook@chromium.org</owner>
   <owner>cros-system-services@google.com</owner>
   <summary>
@@ -915,7 +915,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="ChromeOS.Settings.LoadDocumentTime" units="ms"
-    expires_after="2020-12-01">
+    expires_after="2021-06-01">
   <owner>jamescook@chromium.org</owner>
   <owner>cros-system-services@google.com</owner>
   <summary>