Update the expiration date of Variations.PolicyRestriction

This is still fairly new mechanism and it will be good to continue to observe its adoption for a while longer.

BUG=1149512

Change-Id: Ifb651ef0020068d6107353f7e7a1d48cb15ba742
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550042
Auto-Submit: Julian Pastarmov <pastarmovj@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#832216}

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

@@ -212,7 +212,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Variations.PolicyRestriction"
-    enum="VariationsRestrictionPolicyValues" expires_after="2020-12-31">
+    enum="VariationsRestrictionPolicyValues" expires_after="2021-05-31">
   <owner>pastarmovj@chromium.org</owner>
   <owner>ydago@chromium.org</owner>
   <owner>src/base/metrics/OWNERS</owner>