Making --data-path a subdir of --user-data-dir makes it easier to clean up
data after a benchmark run.

Bug: chromium:861668
Change-Id: If44527163ea396b11346d65d76411d03a5b9a424
Reviewed-on: https://chromium-review.googlesource.com/c/1292065Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56856}