GTTF: make gclient config overwrite the .gclient file if it exists

This prevents bots waiting 20 minutes and failing the update step.
Review URL: http://codereview.chromium.org/7492019

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@93678 0039d316-1c4b-4281-b951-d872f2087c98

gclient.py

@@ -981,9 +981,6 @@ URL.
       (not options.spec and not args)):
     parser.error('Inconsistent arguments. Use either --spec or one or 2 args')
 
-  if os.path.exists(options.config_filename):
-    raise gclient_utils.Error('%s file already exists in the current directory'
-                                  % options.config_filename)
   client = GClient('.', options)
   if options.spec:
     client.SetConfig(options.spec)