Extend the expiry date of URLValidationResult histograms.

Bug: 1144792
Change-Id: I9563b56493a68dc6f7598c2b08248612575f552a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2623195Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Caitlin Fischer <caitlinfischer@google.com>
Cr-Commit-Position: refs/heads/master@{#843327}

tools/metrics/histograms/histograms_xml/variations/histograms.xml

@@ -162,7 +162,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram
     name="Variations.Headers.URLValidationResult{VariationsHeadersURLValidationResult}"
-    enum="VariationsHeadersURLValidationResult" expires_after="2020-12-15">
+    enum="VariationsHeadersURLValidationResult" expires_after="2021-03-15">
   <owner>asvitkine@chromium.org</owner>
   <owner>jwd@chromium.org</owner>
   <owner>src/base/metrics/OWNERS</owner>