Extend histogram expiry

This aligns the expiry of
Navigation.MainFrameFormSubmission.SiteEngagementLevel with
Navigation.MainFrame.SiteEngagementLevel.

Bug: 1140053
Change-Id: I36ba7bdf1980f494fb6798e4a9902e45d67366b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2561700Reviewed-by: Chris Thompson <cthomp@chromium.org>
Commit-Queue: Mustafa Emre Acer <meacer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831941}

tools/metrics/histograms/histograms_xml/navigation/histograms.xml

@@ -761,7 +761,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Navigation.MainFrameFormSubmission.SiteEngagementLevel"
-    enum="SiteEngagementLevel" expires_after="2020-12-01">
+    enum="SiteEngagementLevel" expires_after="2021-04-04">
   <owner>meacer@chromium.org</owner>
   <owner>security-enamel@chromium.org</owner>
   <summary>