Fix regression in webdriver tests

A large number of tests in webdriver directory are currently failing,
due to a change in the parameters of Find.css method by PR web-platform-tests#17260,
which broke any tests that still supply the old parameters.

This commit changes the parameter order and uses default parameter value
to make the parameter list compatible with the old one.

tools/webdriver/webdriver/client.py

@@ -313,7 +313,7 @@ def __init__(self, session):
         self.session = session
 
     @command
-    def css(self, element_selector, frame, all=True):
+    def css(self, element_selector, all=True, frame="window"):
         if (frame != "window"):
             self.session.switch_frame(frame)
         elements = self._find_element("css selector", element_selector, all)

tools/wptrunner/wptrunner/executors/executorwebdriver.py

@@ -143,7 +143,7 @@ def elements_by_selector(self, selector):
         return self.webdriver.find.css(selector)
 
     def elements_by_selector_and_frame(self, element_selector, frame):
-        return self.webdriver.find.css(element_selector, frame)
+        return self.webdriver.find.css(element_selector, frame=frame)
 
 
 class WebDriverClickProtocolPart(ClickProtocolPart):