Fix presubmit @2x recognized as @at-keyword.

chrome/browser/test_presubmit.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """Unit tests for Web Development Style Guide checker."""
 
 import os
 import re
 import sys
@@ skipping 749 matching lines @@
     width: 0px;
   }
   10% {
     width: 10px;
   }
   100% {
     width: 100px;
   }
 }
 
+#logo {
+  background-image: url(images/google_logo.png@2x);
+}
+
+body.alternate-logo #logo {
+  -webkit-mask-image: url(images/google_logo.png@2x);
+}
+
 /* http://crbug.com/359682 */
 #spinner-container #spinner {
   -webkit-animation-duration: 1.0s;
 }
 
 .media-button.play > .state0.active,
 .media-button[state='0'] > .state0.normal /* blah */, /* blee */
 .media-button[state='0']:not(.disabled):hover > .state0.hover {
   -webkit-animation: anim 0s;
   -webkit-animation-duration: anim 0ms;
@@ skipping 30 matching lines @@
     opacity: .0;
     opacity: 0.0;
     opacity: 0.;
     border-width: 0mm;
     height: 0cm;
     width: 0in;
 """)
 
 if __name__ == '__main__':
   unittest.main()