In run-webkit-tests, start xvfb if necessary.

third_party/WebKit/Tools/Scripts/webkitpy/common/system/executive_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 38 matching lines @@
 
     def poll(self):
         # Consider the process completed when all the stdout and stderr has been read.
         if self.stdout.len != self.stdout.tell() or self.stderr.len != self.stderr.tell():
             return None
         return self.returncode
 
     def communicate(self, *_):
         return (self.stdout.getvalue(), self.stderr.getvalue())
 
+    def kill(self):
+        return
+
 
 class MockExecutive(object):
     PIPE = 'MOCK PIPE'
     STDOUT = 'MOCK STDOUT'
     DEVNULL = 'MOCK_DEVNULL'
 
     @staticmethod
     def ignore_error(error):
         pass
 
@@ skipping 128 matching lines @@
 
 
 def mock_git_commands(vals, strict=False):
     def run_fn(args):
         sub_command = args[1]
         if strict and sub_command not in vals:
             raise AssertionError('{} not found in sub-command list {}'.format(
                 sub_command, vals))
         return vals.get(sub_command, '')
     return MockExecutive(run_command_fn=run_fn)