Thus downstream recipes could run "train" without explicit
mock data everywhere.

BUG=1293270
TEST=train
Recipe-Nontrivial-Roll: build
Recipe-Nontrivial-Roll: chromiumos
Recipe-Nontrivial-Roll: build_limited
Recipe-Nontrivial-Roll: chrome_release
Change-Id: I09f6a27097f400dabd990f6ef61e301a75a39691
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3502768Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Xinan Lin <linxinan@chromium.org>