What steps will reproduce the problem?
(1) There is a defunct wptserve process; there is also a pid file in /tmp/WebKit/wptserve.pid
(2) Run run-webkit-tests
(3) Fail to run with the following error

===
14:57:50.489 61907 Starting WPTServe ...
14:57:50.493 61907 stale wptserve pid file, pid 115352
14:57:50.494 61907 Flushing stdout
14:57:50.494 61907 Flushing stderr
14:57:50.494 61907 Stopping helper
14:57:50.494 61907 Cleaning up port

AttributeError raised: 'NoneType' object has no attribute 'wait'
Traceback (most recent call last):
  File "/usr/local/google/home/shimazu/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py", line 69, in main
    return run(port, options, args, stderr, stdout).exit_code
  File "/usr/local/google/home/shimazu/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py", line 553, in run
    run_details = _run_tests(port, options, args, printer)
  File "/usr/local/google/home/shimazu/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/run_webkit_tests.py", line 544, in _run_tests
    return manager.run(args)
  File "/usr/local/google/home/shimazu/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/manager.py", line 130, in run
    self._start_servers(tests_to_run)
  File "/usr/local/google/home/shimazu/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/controllers/manager.py", line 374, in _start_servers
    self._port.start_wptserve()
  File "/usr/local/google/home/shimazu/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/port/base.py", line 1185, in start_wptserve
    server.start()
  File "/usr/local/google/home/shimazu/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/servers/server_base.py", line 93, in start
    self._stop_running_server()
  File "/usr/local/google/home/shimazu/chromium/src/third_party/WebKit/Tools/Scripts/webkitpy/layout_tests/servers/wptserve.py", line 63, in _stop_running_server
    self._process.wait()
AttributeError: 'NoneType' object has no attribute 'wait'

===

What is the expected output?

Showing more specific error like "Fail to kill an existing process.", or kill it correctly.