Fix up histograms for TLS13 Downgrade.

Bug: boringssl:226
Change-Id: I9541ae612322776a7ccddd8e163ab3025dba1e7a
Reviewed-on: https://chromium-review.googlesource.com/1188849Reviewed-by: David Benjamin <davidben@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Steven Valdez <svaldez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586260}

tools/metrics/histograms/histograms.xml

@@ -57648,8 +57648,10 @@ uploading your change for review.
   </summary>
 </histogram>
 
-<histogram name="Net.SSLTLS13DowngradeTLS13Experiment" enum="BooleanDowngrade">
-  <owner>svaldez@chromium.org,davidben@chromium.org</owner>
+<histogram name="Net.SSLTLS13DowngradeTLS13Experiment" enum="BooleanDowngrade"
+    expires_after="2019-03-31">
+  <owner>svaldez@chromium.org</owner>
+  <owner>davidben@chromium.org</owner>
   <summary>
     For each HTTPS connection to the TLS 1.3 experiment server set, whether the
     TLS 1.3 anti-downgrade mechanism would have fired. This is only recorded if