style: Rename -moz-menulist-button to -moz-menulist-arrow-button.
style: Rename -moz-menulist-button to -moz-menulist-arrow-button.

This should be less confusing. This is not supported outside of chrome:// or
user-agent stylesheets so we can name this however we want.

Differential Revision: https://phabricator.services.mozilla.com/D65605

splav/servo,splav/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,splav/servo,KiChjang/servo