kwargs['stdin'] is passed in to subprocess communicate(), which
expects bytes, not str.

Bug: 1215602
Change-Id: I95326eed08cffecd31abc6b0380de2a3c2ff3dd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2929386Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Dan H <harringtond@chromium.org>