Mark an expired histogram as obsolete.

The code that was emitting this histogram has already been removed.

Bug: 995542
Change-Id: I36fbc89b33ba483ecfb3d8b93e54cbbfc8336173
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1823217
Auto-Submit: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699732}

tools/metrics/histograms/histograms.xml

@@ -66616,6 +66616,9 @@ uploading your change for review.
 
 <histogram name="Memory.Experimental.WMIRefresher.InitDiskIdleTimeConfigStatus"
     enum="WMIRefresherInitStatus" expires_after="2019-09-30">
+  <obsolete>
+    Obsolete as of 09/2019
+  </obsolete>
   <owner>sebmarchand@chromium.org</owner>
   <summary>
     The result of configuring the WMIRefresher to read the disk idle time