presubmit_support: Run tests on parallel when asked.
presubmit_support: Run tests on parallel when asked.

Bug: 819774
Change-Id: I19cd59aab23f3afb39da7fa04c470ad4b1c688f4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1627402
Reviewed-by: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Commit-Queue: Edward Lesmes <d05609f027dc667cc37492b1dc1bdea5d60fa82a@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools