Change the way to detect build slave.

It seems sys.stdout.isatty() is not a good signal on the build slaves.

TBR=bradnelson
TEST=none
BUG=none

Review URL: http://codereview.chromium.org/3342028

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@59131 0039d316-1c4b-4281-b951-d872f2087c98

gclient.py

@@ -1176,9 +1176,10 @@ def Main(argv):
         for fn in dir(sys.modules[__name__]) if fn.startswith('CMD')]))
     parser = optparse.OptionParser(version='%prog ' + __version__)
     # TODO(maruel): Temporary workaround to disable parallel checkout on
-    # buildbots until they can correctly parse its output. Uses that fact that
+    # buildbots until they can correctly parse its output. stdout being
-    # stdout is redirected as a signal.
+    # redirected is not a good signal, at least on linux. Check for environment
-    if sys.stdout.isatty():
+    # variable instead.
+    if 'CHROME_HEADLESS' not in os.environ:
       jobs = 8
     else:
       jobs = 1