Fix bug in os_utilities.get_gpu() on cygwin.
Fix bug in os_utilities.get_gpu() on cygwin.

It would cause a crash.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.appspot.com/191040043

madecoste/swarming,madecoste/swarming,pombreda/swarming,pombreda/swarming,madecoste/swarming,madecoste/swarming,pombreda/swarming,luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py,pombreda/swarming