This patch changes how we deal with window control buttons on left
layouts. Previously, we looked to a pref that was only set on macos
but with this patch we use media queries to detect whether the system
is macOS or gtk-csd buttons use left-side layout.
To support old-style "override pref" this patch also introduces a
new custom pref userchrome.force-window-controls-on-left.enabled
which can be used e.g on Windows to move window controls to left side.
This logic is currently only supported in styles that use
window_control_placeholder_support.css
/* Use this pref to check Mac OS where window controls are on left */
/* Rules for window controls on left layout */
/* This pref defaults to true on Mac and doesn't actually do anything on other platforms. So if your system has window controls on LEFT side you can set the pref to true */