Now that CSE uses dom tree to order partitions, we need the
dom tree computed before benchmarking CSE.

Fixes #15801

Change-Id: Ifa4702c7b75250f34de185e69a880b3f3cc46a12
Reviewed-on: https://go-review.googlesource.com/23361Reviewed-by: David Chase <drchase@google.com>