Mark Renderer.CompositedScrolling histogram obsolete.

The reporting code no longer exists, and we haven't noticed missing this
histogram so it should be safe to expire.

Bug: 1089338
Change-Id: Iab50286cbdb4ddb3d403df83b6bfddafc50dd236
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2640750Reviewed-by: Glenn Hartmann <hartmanng@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Robert Flack <flackr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845650}

tools/metrics/histograms/histograms_xml/renderer/histograms.xml

@@ -23,6 +23,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="Renderer.CompositedScrolling" enum="CompositedScrolling"
     expires_after="M85">
+  <obsolete>
+    Expired in M86.
+  </obsolete>
   <owner>hartmanng@chromium.org</owner>
   <summary>
     Total count of the number of RenderLayers which are scrollable areas, need