Extend SBClientPhishing.VisualComparisonTime

This metric is related to the unlaunched feature:
https://bugs.chromium.org/p/chromium/issues/detail?id=1066665

It will be useful until the feature is landed.

Fixed: 1138394
Change-Id: I5ec6de40f12714f52bf7cda41394d2e863cb4190
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472605
Auto-Submit: Daniel Rubery <drubery@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817685}

tools/metrics/histograms/histograms_xml/sb_client/histograms.xml

@@ -571,7 +571,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="SBClientPhishing.VisualComparisonTime" units="ms"
-    expires_after="2020-11-22">
+    expires_after="2021-05-22">
   <owner>drubery@chromium.org</owner>
   <owner>chrome-safebrowsing-alerts@google.com</owner>
   <summary>