Aggregate Blink.UseCounter metrics.

Current enumeration support ignores pages that don't emit any
of the desired metric so technically this will be proportion of
pages that use Blink instead of the proportion of all pages.

However, since it seems likely that the number of pages that don't
use any Blink features is incredibly small (if not outright zero),
this is acceptable for now.

Note: A recent CL will make a huge difference with the per-page
proportions so avoid looking at M72 Canary/Dev aggregations.  M73
or M72 Beta/Stable (once released) will be fine.
https://chromium-review.googlesource.com/c/chromium/src/+/1343060

Change-Id: I46b3092caa0a21e1c5e6baac9977b0efc8ae39e4
Reviewed-on: https://chromium-review.googlesource.com/c/1355486Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Luna Lu <loonybear@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613083}

tools/metrics/ukm/ukm.xml

@@ -900,6 +900,13 @@ be describing additional metrics about the same event.
     <summary>
       Opt-in UseCounter feature.
     </summary>
+    <aggregation>
+      <history>
+        <statistics>
+          <enumeration/>
+        </statistics>
+      </history>
+    </aggregation>
   </metric>
 </event>