Treat cygwin the same as Windows when selecting the GN binary.

R=maruel@chromium.org

Review URL: https://codereview.chromium.org/106243002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239028 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239028