Set the default device_serial argument (#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 (#31634)
(#31634)