title: Bokeh throws a RuntimeError whenever I try to run export_png
body: <p>I just updated all of my python packages and some of my scripts aren't working properly. Specifically, Bokeh is having issues with the <code>export_png</code> function. </p>
  
  <p>Normally that just saves a copy of my chart to a specific directory. Now I'm getting this error:</p>
  
  <pre><code>Traceback (most recent call last):
  
    File "C:\Users\ethan\projects\health\chart.py", line 112, in &lt;module&gt;
      p = column(make_chart(False), make_chart(True))
  
    File "C:\Users\ethan\projects\health\chart.py", line 106, in make_chart
      export_png(p, filename = name + '.png')
  
    File "C:\ProgramData\Anaconda3\lib\site-packages\bokeh\io\export.py", line 97, in export_png
      image = get_screenshot_as_png(obj, width=width, height=height, driver=webdriver, timeout=timeout)
  
    File "C:\ProgramData\Anaconda3\lib\site-packages\bokeh\io\export.py", line 201, in get_screenshot_as_png
      web_driver = driver if driver is not None else webdriver_control.get()
  
    File "C:\ProgramData\Anaconda3\lib\site-packages\bokeh\io\webdriver.py", line 118, in get
      self.current = self.create()
  
    File "C:\ProgramData\Anaconda3\lib\site-packages\bokeh\io\webdriver.py", line 122, in create
      driver = self._create(kind)
  
    File "C:\ProgramData\Anaconda3\lib\site-packages\bokeh\io\webdriver.py", line 140, in _create
      raise RuntimeError("Neither firefox and geckodriver nor a variant of chromium browser and " \
  
  RuntimeError: Neither firefox and geckodriver nor a variant of chromium browser and chromedriver are available on system PATH. You can install the former with 'conda install -c conda-forge firefox geckodriver'.
  </code></pre>
  
  <p>Chrome works fine on my computer I'm using it now. I'm not sure why bokeh needs to use a browser for this because I'm just making a png file but is there a way to fix this without installing firefox? I'd prefer to keep using chrome. </p>