Add IOS.URLMismatchInLegacyAndSlimNavigationManager to History Table.

Use enumeration statistic type and no index fields.

Bug: 1182320
Change-Id: If45573cd4572fd9ed634710a026ac898e001a0cf
Reviewed-on: https://chromium-review.googlesource.com/c/1354329Reviewed-by: Brian White <bcwhite@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612279}

tools/metrics/ukm/ukm.xml

@@ -5677,6 +5677,13 @@ be describing additional metrics about the same event.
     <summary>
       True if there was a mismatch.
     </summary>
+    <aggregation>
+      <history>
+        <statistics>
+          <enumeration/>
+        </statistics>
+      </history>
+    </aggregation>
   </metric>
 </event>