[py] Re-add defaults for Chromium kwargs (#16372)

Author: cgoldberg

py/selenium/webdriver/chromium/webdriver.py

@@ -31,8 +31,8 @@ class ChromiumDriver(RemoteWebDriver):
 
     def __init__(
         self,
-        browser_name: str,
-        vendor_prefix: str,
+        browser_name: Optional[str] = None,
+        vendor_prefix: Optional[str] = None,
         options: Optional[ChromiumOptions] = None,
         service: Optional[ChromiumService] = None,
         keep_alive: bool = True,