test_bug159346.xhtml modifies UA stylesheet to show scrollbar button.
However, on Ubuntu 18.04, its width is 0. Therefore, the following
script fails to test click on the button.
This patch specifies its `min-width` and `min-height` to make it
clickable on any platforms.
Differential Revision: https://phabricator.services.mozilla.com/D58189
--HG--
extra : moz-landing-system : lando
