The change I landed in 61bf638 couldn't include the proper tests
because of crbug.com/1201918 (I think), and so I split the
test file out into this separate follow-up CL.

Bug: 1157663
Change-Id: I06697a5b76f81f0dc96ad457406a8d4d942d882c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2846635Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>