The vpython/CIPD work created some errors when attempting to build on
Cygwin. This patch fixes these errors by converting Cygwin paths to
Windows paths or using the depot_tools python instead of Cygwin python.

Bug:
Change-Id: Id62f59a357fa2937c7eb843bf306ef4303b0910e
Reviewed-on: https://chromium-review.googlesource.com/745727Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>