new-run-webkit-tests --gtk fails with "AttributeError: GtkDr

$ new-run-webkit-tests --gtk
Starting testing ...Process _Process-1:
Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/manager_worker_broker.py", line 267, in run
    self._client.run(port_obj)
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/worker.py", line 104, in run
    self.cleanup()
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/worker.py", line 135, in cleanup
    self.kill_driver()
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/worker.py", line 159, in kill_driver
    self._driver.stop()
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/port/gtk.py", line 57, in stop
    webkit.WebKitDriver.stop(self)
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/port/webkit.py", line 635, in stop
    if self._server_process:
AttributeError: GtkDriver instance has no attribute '_server_process'
Process _Process-2:
Traceback (most recent call last):
  File "/usr/lib/python2.7/multiprocessing/process.py", line 258, in _bootstrap
    self.run()
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/manager_worker_broker.py", line 267, in run
    self._client.run(port_obj)
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/worker.py", line 104, in run
    self.cleanup()
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/worker.py", line 135, in cleanup
    self.kill_driver()
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/worker.py", line 159, in kill_driver
    self._driver.stop()
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/port/gtk.py", line 57, in stop
    webkit.WebKitDriver.stop(self)
  File "/home/vanya/git-repos/opensource/webkit/WebKit/Tools/Scripts/webkitpy/layout_tests/port/webkit.py", line 635, in stop
    if self._server_process:
AttributeError: GtkDriver instance has no attribute '_server_process'
worker/0 raised OSError('[Errno 2] No such file or directory'):
  layout_tests/controllers/worker.py:91 (in run)
    self._worker_connection.run_message_loop()
  layout_tests/controllers/message_broker.py:191 (in run_message_loop)
    self._broker.run_message_loop(self._run_topic, self._client, delay_secs)
  layout_tests/controllers/message_broker.py:127 (in run_message_loop)
    self._run_loop(topic_name, client, block=True, delay_secs=delay_secs)
  layout_tests/controllers/message_broker.py:141 (in _run_loop)
    self._dispatch_message(msg, client)
  layout_tests/controllers/message_broker.py:150 (in _dispatch_message)
    message_handler(message.src, *optargs)
  layout_tests/controllers/worker.py:111 (in handle_test_list)
    self._run_test(test_input)
  layout_tests/controllers/worker.py:126 (in _run_test)
    result = self.run_test_with_timeout(test_input, test_timeout_sec)
  layout_tests/controllers/worker.py:165 (in run_test_with_timeout)
    return self._run_test_in_this_thread(test_input)
  layout_tests/controllers/worker.py:249 (in _run_test_in_this_thread)
    self._driver.start()
  layout_tests/port/gtk.py:48 (in start)
    self._xvfb_process = subprocess.Popen(run_xvfb, stderr=devnull)
  /usr/lib/python2.7/subprocess.py:679 (in __init__)
    errread, errwrite)
  /usr/lib/python2.7/subprocess.py:1239 (in _execute_child)
    raise child_exception