use a single driver argument

Author: Conengmo

folium/folium.py

@@ -297,28 +297,28 @@ def _repr_html_(self, **kwargs):
             out = self._parent._repr_html_(**kwargs)
         return out
 
-    def _to_png(self, delay=3, webdriver_type='firefox', webdriver_path=None):
+    def _to_png(self, delay=3, driver=None):
         """Export the HTML to byte representation of a PNG image.
 
         Uses selenium to render the HTML and record a PNG. You may need to
         adjust the `delay` time keyword argument if maps render without data or tiles.
 
+        Uses a headless Firefox webdriver by default, though you can provide your own.
+
         Examples
         --------
         >>> m._to_png()
         >>> m._to_png(time=10)  # Wait 10 seconds between render and snapshot.
 
         """
         if self._png_image is None:
-            from selenium import webdriver
-            if webdriver_type == "chrome":
-                options = webdriver.ChromeOptions()
-                options.add_argument('--headless')
-                driver = webdriver.Chrome(webdriver_path, options=options)
-            else:
+            if driver is None:
+                from selenium import webdriver
+
                 options = webdriver.firefox.options.Options()
                 options.add_argument('--headless')
                 driver = webdriver.Firefox(options=options)
+
             html = self.get_root().render()
             with temp_html_filepath(html) as fname:
                 # We need the tempfile to avoid JS security issues.