[web_dev_style] Changing :pseudo-element to ::pseudo-element to match

chrome/browser/resources/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 431 matching lines @@
 
 a,
 div,a {
   some-other: rule here;
 }""", """
 - One selector per line (what not to do: a, b {}).
     div,a,
     div, span,
     div,a {""")
 
+  def testCssPseudoElementDoubleColon(self):
+    self.VerifyContentsProducesOutput("""
+a:href,
+br::after,
+::-webkit-scrollbar-thumb,
+a:not([empty]):hover:focus:active, /* shouldn't catch here and above */
+abbr:after,
+.tree-label:empty:after,
+b:before,
+:-webkit-scrollbar {
+  rule: value;
+}""", """
+- Pseudo-elements should use double colon (i.e. ::after).
+    :after (should be ::after)
+    :after (should be ::after)
+    :before (should be ::before)
+    :-webkit-scrollbar (should be ::-webkit-scrollbar)
+    """)
+
   def testCssRgbIfNotGray(self):
     self.VerifyContentsProducesOutput("""
 #abc,
 #aaa,
 #aabbcc {
   background: -webkit-linear-gradient(left, from(#abc), to(#def));
   color: #bad;
   color: #bada55;
 }""", """
 - Use rgb() over #hex when not a shade of gray (like #333).
@@ skipping 54 matching lines @@
     opacity: .0;
     opacity: 0.0;
     opacity: 0.;
     border-width: 0mm;
     height: 0cm;
     width: 0in;
 """)
 
 if __name__ == '__main__':
   unittest.main()