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

mojo/tools/mojob.py

Index: mojo/tools/mojob.py
diff --git a/mojo/tools/mojob.py b/mojo/tools/mojob.py
index d0505b8fcc443099d8b65c9ed1b994335465e9d8..4ac1de5e1df1f0dcf5fc20498dfda28559636105 100755
--- a/mojo/tools/mojob.py
+++ b/mojo/tools/mojob.py
@@ -8,6 +8,7 @@
 import argparse
 import os
 import platform
+import re
 import subprocess
 import sys
 
@@ -71,11 +72,13 @@ def gn(args):
 
 
 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):