fix virtual_test_suites() for browser_test ports

Tools/Scripts/webkitpy/layout_tests/port/mock_drt_unittest.py

 # Copyright (C) 2011 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 59 matching lines @@
 
     def test_uses_apache(self):
         pass
 
     def test_get_crash_log(self):
         pass
 
     def test_check_build(self):
         pass
 
+    def test_virtual_test_suites(self):
+        pass
+
 
 class MockDRTTest(unittest.TestCase):
     def input_line(self, port, test_name, pixel_tests, checksum=None):
         url = port.create_driver(0).test_to_uri(test_name)
         if url.startswith('file://'):
             url = url[len('file://'):]
         if pixel_tests:
             url += "'--pixel-test"
         if checksum:
             url += "'" + checksum
@@ skipping 100 matching lines @@
         self.assertTest('passes/audio.html', pixel_tests=True,
                         drt_output=['Content-Type: audio/wav\n',
                          'Content-Transfer-Encoding: base64\n',
                          'YXVkaW8td2F2',
                          '\n',
                          '#EOF\n',
                          '#EOF\n'])
 
     def test_virtual(self):
         self.assertTest('virtual/passes/text.html', True)