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

build/gyp_chromium

 #!/usr/bin/env python
 
 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script is wrapper for Chromium that adds some support for how GYP
 # is invoked by Chromium beyond what can be done in the gclient hooks.
 
 import glob
@@ skipping 124 matching lines @@
 
   return result
 
 
 def RunGN(supplemental_includes):
   """Runs GN, returning True if it succeeded, printing an error and returning
   false if not."""
 
   # The binaries in platform-specific subdirectories in src/tools/gn/bin.
   gnpath = SRC_DIR + '/tools/gn/bin/'
-  if sys.platform == 'win32':
+  if sys.platform in ('cygwin', 'win32'):
     gnpath += 'win/gn.exe'
   elif sys.platform.startswith('linux'):
     # On Linux we have 32-bit and 64-bit versions.
     if subprocess.check_output(["getconf", "LONG_BIT"]).find("64") >= 0:
       gnpath += 'linux/gn'
     else:
       gnpath += 'linux/gn32'
   elif sys.platform == 'darwin':
     gnpath += 'mac/gn'
   else:
@@ skipping 122 matching lines @@
   # to enfore syntax checking.
   syntax_check = os.environ.get('CHROMIUM_GYP_SYNTAX_CHECK')
   if syntax_check and int(syntax_check):
     args.append('--check')
 
   print 'Updating projects from gyp files...'
   sys.stdout.flush()
 
   # Off we go...
   sys.exit(gyp.main(args))