Extend dialog scheme UMAs

Removing cross-origin dialogs is being worked on in
https://chromestatus.com/feature/5148698084376576. Make sure
that relevant UMAs don't expire.

Fixed: 1138390
Bug: 1065085
Change-Id: I6cb9c05bb838e065ff98e7b4af9f2d9fcc5adf1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2472481
Auto-Submit: Avi Drissman <avi@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817284}

tools/metrics/histograms/histograms_xml/others/histograms.xml

@@ -7023,9 +7023,10 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram base="true" name="JSDialogs.Scheme" enum="NavigationScheme"
-    expires_after="2020-11-15">
+    expires_after="2021-06-01">
   <owner>avi@chromium.org</owner>
   <owner>carlosil@chromium.org</owner>
+  <owner>ericmill@google.com</owner>
   <owner>meacer@chromium.org</owner>
   <summary>The scheme of the URL showing a JavaScript dialog.</summary>
 </histogram>