Restore UKM field dropped by a revert.

This field was dropped by a revert, but we have data for it, so re-add the description.
https://crrev.com/d38f4e3c1225c36574911f6fdeecdb1b236dd054

Change-Id: Ie68bc76ced75a1d6269b252801cda76baa5de9ff
Reviewed-on: https://chromium-review.googlesource.com/1188765Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#586064}

tools/metrics/ukm/ukm.xml

@@ -3644,6 +3644,12 @@ be describing additional metrics about the same event.
     Collects the duration in MS from when the tab is backgrounded to when it is
     brought to foreground or closed.
   </summary>
+  <metric name="IsDiscarded">
+    <summary>
+      Boolean value indicating whether or not the tab is discarded since it was
+      backgrounded.
+    </summary>
+  </metric>
   <metric name="IsForegrounded">
     <summary>
       Boolean value indicating whether or not it's triggered because the tab is