text: Title: browser.close() raises error message when `userDataDir` option is set
  Body: I'm using latest version of pyppeteer (0.2.2 from `dev` branch) on Windows 10.
  
  Minimal code to reproduce the issue:
  
  ```python
  import asyncio
  from pyppeteer import launch
  
  async def test():
      browser = await launch({'userDataDir': 'test'})
      await browser.close()
  
  asyncio.run(test())
  ```
  
  And here is the error message:
  
  ```
  Error in atexit._run_exitfuncs:
  Traceback (most recent call last):
    File "C:\[ projects ]\kroger-cli\venv\lib\site-packages\pyppeteer\launcher.py", line 151, in _close_process
      self._loop.run_until_complete(self.killChrome())
    File "D:\Python\Python38\lib\asyncio\base_events.py", line 591, in run_until_complete
      self._check_closed()
    File "D:\Python\Python38\lib\asyncio\base_events.py", line 508, in _check_closed
      raise RuntimeError('Event loop is closed')
  RuntimeError: Event loop is closed
  sys:1: RuntimeWarning: coroutine 'Launcher.killChrome' was never awaited
  RuntimeWarning: Enable tracemalloc to get the object allocation traceback
  ```
  
  Removing the `userDataDir` option from the launch eliminates the issue (i.e. everything is working as expected).
  
  Is it something you can help with? Happy to provide any logs/details that could help. Thanks!