Update update_depot_tools to always check out Windows tools on Cygwin.
Update update_depot_tools to always check out Windows tools on Cygwin.

This is necessary so that Cygwin users have a python directory in depot_tools/,
which will soon be used by gyp_chromium.

BUG=335180

Review URL: https://codereview.chromium.org/141523009

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@245704 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools