Port get_gn_arg_value() to native Python code so that it works on Windows.

mojo/tools/mojob.py

 #!/usr/bin/env python
 # Copyright 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.
 
 # This a simple script to make building/testing Mojo components easier.
 
 import argparse
 import os
 import platform
+import re
 import subprocess
 import sys
 
 import mopy.paths
 
 
 def get_out_dir(args):
   out_dir = "out"
   prefix = ''
   if args.android:
@@ skipping 43 matching lines @@
 
   out_dir = get_out_dir(args)
   command.append(out_dir)
   command.append('--args="%s"' % ' '.join(gn_args))
 
   print 'Running %s ...' % ' '.join(command)
   return subprocess.call(' '.join(command), shell=True)
 
 
 def get_gn_arg_value(out_dir, arg):
-  if platform.system() == 'Windows':
-    return None  # TODO(jam): implement
-  command = (r'''grep -m 1 "^[[:space:]]*\<%s\>" "%s/args.gn" |
-      sed -n 's/.* = "\?\([^"]*\)"\?$/\1/p' ''') % (arg, out_dir)
-  return subprocess.check_output(command, shell=True).strip()
+  key_value_regex = re.compile(r'^%s = (.+)$' % arg)
+  with open(os.path.join(out_dir, "args.gn"), "r") as args_file:
+    for line in args_file.readlines():
+      m = key_value_regex.search(line)
+      if m:
+        return m.group(1).strip('"')
+  return ''
 
 
 def build(args):
   out_dir = get_out_dir(args)
   print 'Building in %s ...' % out_dir
   if get_gn_arg_value(out_dir, 'use_goma') == 'true':
     # Use the configured goma directory.
     local_goma_dir = get_gn_arg_value(out_dir, 'goma_dir')
     print 'Ensuring goma (in %s) started ...' % local_goma_dir
     command = [os.path.join(local_goma_dir, 'goma_ctl.py'), 'ensure_start']
@@ skipping 160 matching lines @@
     args.clang = False
 
   if platform.system() == 'Windows':
     args.clang = False
 
   return args.func(args)
 
 
 if __name__ == '__main__':
   sys.exit(main())