Bug 1710910 - Reduce the size of the close icon for infobars and use the...

Bug 1710910 - Reduce the size of the close icon for infobars and use the correct smaller close icon for tabs r=mstriemer,desktop-theme-reviewers,dao

* Original author was jaws, prathikshaprasadsuman got it reviewed and landed
* Update to non-hovered .tab-close-button width to avoid regressing bug 1818233 again.

Differential Revision: https://phabricator.services.mozilla.com/D115006