[metrics] Mark unused V8.DetachedContextAgeInGC as obsolete

Bug: 677883
Change-Id: I7d594c9046ca8ae9c94a4be83aee92580f3c4edb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710321Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691663}

tools/metrics/histograms/histograms.xml

@@ -148018,6 +148018,9 @@ regressions. -->
 </histogram>
 
 <histogram name="V8.DetachedContextAgeInGC">
+  <obsolete>
+    This histogram has been removed.
+  </obsolete>
   <owner>ulan@chromium.org</owner>
   <summary>
     Number of garbage collections that a detached global context survives,