Add dominant/latest version index to PaintTiming.NavigationToFirstContentfulPaint UKM event.

Change-Id: Ib63a04cc663011872f60bd04be420b3207dffb29
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869451
Auto-Submit: Brian White <bcwhite@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Robert Kaplow <rkaplow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#708206}

tools/metrics/ukm/ukm.xml

@@ -5943,6 +5943,8 @@ be describing additional metrics about the same event.
     <aggregation>
       <history>
         <index fields="profile.country"/>
+        <index fields="profile.is_dominant_version"/>
+        <index fields="profile.is_latest_version"/>
         <statistics>
           <quantiles type="std-percentiles"/>
         </statistics>