get_toolchain_if_necessary.py contains a number of Windows-isms, including
computing hashes on Windows-style path names, so does not work under
Cygwin. This change reruns it under depot_tools' Windows Python if run
from Cygwin Python.

Review URL: https://codereview.chromium.org/233563003

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@263301 0039d316-1c4b-4281-b951-d872f2087c98