Add sky_server for mac host

sky/tools/download_sky_server

 #!/usr/bin/env python
 # Copyright (c) 2014 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.
 
 import os
 import stat
 import subprocess
+import sys
 
 from webkitpy.common.system import filesystem
 from webkitpy.common.webkit_finder import WebKitFinder
 
+PLATFORM_MAPPING = {
+    'cygwin': 'win',
+    'darwin': 'mac',
+    'linux2': 'linux',
+    'win32': 'win',
+}
+
+sha1 = 'sky_server_' + PLATFORM_MAPPING[sys.platform] + '.sha1'
+
 finder = WebKitFinder(filesystem.FileSystem())
 
 output_file_ = finder.path_from_chromium_base('out', 'downloads', 'sky_server')
 subprocess.call([
     'download_from_google_storage',
     '--no_resume',
     '--no_auth',
     '--bucket', 'mojo',
-    '--sha1_file', finder.path_from_chromium_base('sky', 'tools', 'skygo', 'sky_server.sha1'),
+    '--sha1_file', finder.path_from_chromium_base('sky', 'tools', 'skygo', sha1),
     '--output', output_file_
 ])
 os.chmod(output_file_, os.stat(output_file_).st_mode | stat.S_IEXEC)