Handle button box in intrinsic text field sizing.
See the previous patch commit message for the desired behavior. Keep
non-explicit sizing working the same, but make explicit sizing account
for the button box size. This matches other browsers, and generally
makes sure that the specified number of columns fit.
Unfortunately this is not testable by WPT because spinners are not
cross-platform / specified anywhere, but I added a Gecko WPT.
Differential Revision: https://phabricator.services.mozilla.com/D215440
bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=1905743
gecko-commit: fef6d2c06ec0b06759a58b3f6cbddd58a4bb7a28
gecko-reviewers: dholbert