This CL allows users to pass the root solution revision to the bot
update step for presubmits. The presubmit for some child repositories
of src have dependencies on src. We need to make sure that the same
branch of a src and child repositories are checked out.

Bug:1238418
Change-Id: If2ea3962d764bddd17b3d17d20e7c644f4065fe0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3085094Reviewed-by: Gavin Mak <gavinmak@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Garrett Beaty <gbeaty@chromium.org>
Commit-Queue: Rakib Hasan <rmhasan@google.com>