Remove the overridden _stop_running_server method in WPTServe.

third_party/WebKit/Tools/Scripts/webkitpy/common/system/system_host_mock.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 51 matching lines @@
         self.stderr = StringIO()
         self.environ = {
             'MOCK_ENVIRON_COPY': '1',
             'PATH': '/bin:/mock/bin'
         }
         self.time_return_val = time_return_val
 
     def time(self):
         return self.time_return_val
 
+    def sleep(self, seconds):
+        self.time_return_val += seconds
+
     def print_(self, *args, **kwargs):
         sep = kwargs.get('sep', ' ')
         end = kwargs.get('end', '\n')
         stream = kwargs.get('stream', self.stdout)
         stream.write(sep.join([str(arg) for arg in args]) + end)