Download: Keep a download core database metric.

Keep metric "Download.Database.IsAvailable" which is helpful to detect
critical issue in download sqlite db.

Bug: 1088728
Change-Id: I63be65de5529a1666dd3edfde9a545279da711b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363668Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Xing Liu <xingliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#815318}

tools/metrics/histograms/histograms_xml/download/histograms.xml

@@ -113,8 +113,11 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="Download.Database.IsAvailable" enum="BooleanAvailable"
-    expires_after="M85">
+    expires_after="never">
+<!-- expires-never: Helpful to track download database issue. -->
+
   <owner>xingliu@chromium.org</owner>
+  <owner>qinmin@chromium.org</owner>
   <summary>
     Records whether the download database is available when database startup
     completes, before starting any pending downloads. If the database is