Fix Attributerrror for Chromium browser (web-platform-tests#33912)

self.last_url_used is compared with None but is never initialized to
None in class Chromium. This means that if the comparison is performed
before self.last_url_used is set to a valid value, wpt will crash with
message

AttributeError: 'Chromium' object has no attribute 'last_url_used'

This problem can be reproduced with this command line:

./wpt run --binary /usr/bin/chromium chromium

An alternate way to fix this would be to add a constructor to Chromium
class that initializes self.last_url_used, but this constructor will
also have to declare and pass parameters to Chromium's base class(es),
that will look more complicated compared to this patch.

tools/wpt/browser.py

@@ -789,7 +789,7 @@ def _get_webdriver_url(self, version):
         # Make sure we use the same revision in an invocation.
         # If we have a url that was last used successfully during this run,
         # that url takes priority over trying to form another.
-        if self.last_url_used is not None:
+        if hasattr(self, "last_url_used") and self.last_url_used is not None:
             return f"{self.last_url_used}{filename}"
 
         return f"{self._get_chromium_download_url(version)}{filename}"