[BackgroundSync] Extend histogram

Bug: 1104925
Change-Id: Ic128c5577ce8a9c079bab701630a1680cbb3380c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2517723
Auto-Submit: Mugdha Lakhani <nator@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824003}

tools/metrics/histograms/histograms_xml/background/histograms.xml

@@ -232,7 +232,7 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 </histogram>
 
 <histogram name="BackgroundSync.Event.FromWakeupTask"
-    enum="BackgroundSyncWakeupTask" expires_after="M86">
+    enum="BackgroundSyncWakeupTask" expires_after="2021-07-31">
   <owner>nator@chromium.org</owner>
   <owner>rayankans@chromium.org</owner>
   <summary>