[ukm] Mark a UKM obsolete

This is a follow-up to the CL:
https://chromium-review.googlesource.com/c/chromium/src/+/2062752

In that CL, I should have mark the Throughput ukm obsolete but forgot
to do it.

Bug: 1048780
Change-Id: I5e060b0b13ddf3bbdcee58e807da3cf0a15d9c92
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2079251Reviewed-by: Bryan McQuade <bmcquade@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746013}

tools/metrics/ukm/ukm.xml

@@ -4289,6 +4289,9 @@ be describing additional metrics about the same event.
 </event>
 
 <event name="Graphics.Smoothness.Throughput">
+  <obsolete>
+    Deprecated 02/2020.
+  </obsolete>
   <owner>animations-dev@chromium.org</owner>
   <owner>sadrul@chromium.org</owner>
   <summary>