Mark DefaultBrowser.InteractionResult as obsolete.

It was removed back in r686270.

BUG=1088705
R=chromium-metrics-reviews@google.com

Change-Id: Ic5e1858802599120f75e3b7c4bacd690a8ed0621
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2438419
Auto-Submit: Greg Thompson <grt@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#812004}

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

@@ -3232,6 +3232,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="DefaultBrowser.InteractionResult"
     enum="MakeChromeDefaultResult" expires_after="M85">
+  <obsolete>
+    Removed in Chrome 78.
+  </obsolete>
   <owner>gab@chromium.org</owner>
   <summary>
     Results of interaction with set-as-default dialog in Win8+ first run flow.