'aggregation' directive for CrossOriginFetchFromContentScript3 UKM event

Bug: 1057232
Change-Id: I56ec2f8d353bc93ccbebed6801d3b826b3c6c8ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2121268Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/master@{#754509}

tools/metrics/ukm/ukm.xml

@@ -3701,6 +3701,13 @@ be describing additional metrics about the same event.
       make it easier to work with queries / dashboards for the overall
       CrossOriginFetchFromContentScript3 event.
     </summary>
+    <aggregation>
+      <history>
+        <statistics>
+          <enumeration/>
+        </statistics>
+      </history>
+    </aggregation>
   </metric>
 </event>