Just remove --css-mixins: {...}; in presubmit checked contents

chrome/browser/web_dev_style/css_checker_test.py

 #!/usr/bin/env python
 # Copyright 2015 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 css_checker
 from os import path as os_path
 import re
 from sys import path as sys_path
 import unittest
@@ skipping 130 matching lines @@
 /* Hello! */div/* Comment here*/{
   display: block;
 }
 
 blah /* hey! */
 {
   rule: value;
 }
 
 .mixed-in {
+  display: none;
   --css-mixin: {
     color: red;
   };  /* This should be ignored. */
 }
 
 .this.is { /* allowed */
   rule: value;
 }""", """
 - Start braces ({) end a selector, have a space before them and no rules after.
     div{
@@ skipping 265 matching lines @@
     opacity: 0.0;
     opacity: 0.;
     border-width: 0mm;
     height: 0cm;
     width: 0in;
 """)
 
 
 if __name__ == '__main__':
   unittest.main()