Set the default device_serial argument (web-platform-tests#45819)

It appears that WPT does not run on the Chrome Android browser
of the simulator unless the '--device-serial' argument is
explicitly added, after the PR[1] that introduced support for
multiple emulators. The CL only sets the default value(emulator-5554)
to Firefox Android. So this CL sets the default value to Chrome
Android browsers as well.

[1] Support multiple emulators for Android browsers (web-platform-tests#31634)
    (web-platform-tests#31634)

tools/wpt/run.py

@@ -565,6 +565,8 @@ def setup_kwargs(self, kwargs):
         if kwargs["package_name"] is None:
             kwargs["package_name"] = self.browser.find_binary(
                 channel=browser_channel)
+        if not kwargs["device_serial"]:
+            kwargs["device_serial"] = ["emulator-5554"]
         if kwargs["webdriver_binary"] is None:
             webdriver_binary = None
             if not kwargs["install_webdriver"]: