This patch implements -moz-gtk-csd-hide-titlebar-by-default media query
to check if the system titlebar should be disabled by default on Linux systems
(it's already disabled on Window/Mac).
It also removes explicit definition of browser.tabs.drawInTitlebar preference on Linux.
When browser.tabs.drawInTitlebar is missing the -moz-gtk-csd-hide-titlebar-by-default
is used to obtain the titlebar state. When browser.tabs.drawInTitlebar is set
in about:config or by Customize menu, the user peference is used instead of the default.
It also fixes a -moz-gtk-csd-available media query,
it was always true regardless the actual system setting.
Differential Revision: https://phabricator.services.mozilla.com/D16036
7537bb8b84