fix virtual_test_suites() for browser_test ports

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

 # Copyright (C) 2014 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 53 matching lines @@
     def driver_name(self):
         return 'browser_tests'
 
     def default_timeout_ms(self):
         timeout_ms = 10 * 1000
         if self.get_option('configuration') == 'Debug':  # pylint: disable=E1101
             # Debug is usually 2x-3x slower than Release.
             return 3 * timeout_ms
         return timeout_ms
 
+    def virtual_test_suites(self):
+        return []
+
 
 class BrowserTestLinuxPort(BrowserTestPortOverrides, linux.LinuxPort):
     pass
 
 
 class BrowserTestMacPort(BrowserTestPortOverrides, mac.MacPort):
     def _path_to_driver(self, configuration=None):
         return self._build_path_with_configuration(configuration, self.driver_name())
 
     def default_timeout_ms(self):
         timeout_ms = 20 * 1000
         if self.get_option('configuration') == 'Debug':  # pylint: disable=E1101
             # Debug is usually 2x-3x slower than Release.
             return 3 * timeout_ms
         return timeout_ms
 
 
 class BrowserTestWinPort(BrowserTestPortOverrides, win.WinPort):
     def default_timeout_ms(self):
         timeout_ms = 20 * 1000
         if self.get_option('configuration') == 'Debug':  # pylint: disable=E1101
             # Debug is usually 2x-3x slower than Release.
             return 3 * timeout_ms
         return timeout_ms