histograms: Change DailyUseTime to not expire.

The Platform.DailyUseTime metric is used to monitor system health on
numerous dashboards, and as such it should not expire.

BUG=chromium:1089270
TEST=CQ

Change-Id: I28d8593a3cf935d4ba7c1b8f4d76d521f8f0b495
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2541784Reviewed-by: Ian Barkley-Yeung <iby@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Commit-Queue: Miriam Zimmerman <mutexlox@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827944}

tools/metrics/histograms/histograms_xml/platform/histograms.xml

@@ -287,12 +287,13 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
   </summary>
 </histogram>
 
-<histogram name="Platform.DailyUseTime" units="seconds" expires_after="M90">
-<!-- TODO(mutexlox): See if this is still in use and find owners -->
+<histogram name="Platform.DailyUseTime" units="seconds" expires_after="never">
+<!-- expires-never: Used to assess Chrome OS platform stability. -->
 
   <owner>semenzato@chromium.org</owner>
   <owner>bsimonnet@chromium.org</owner>
   <owner>mutexlox@chromium.org</owner>
+  <owner>iby@chromium.org</owner>
   <owner>cros-telemetry@google.com</owner>
   <summary>
     Total use time (device ON and not asleep) in the previous day, or the most