Bug 1812868 - Expose scrollbar-inline-size as a CSS variable to chrome code. r=mstange

For that we need to:

 * Make GetDPIRatioForScrollbarPart thread-safe: This was using the
   widget for bug 1727289, but just looking at the print preview scale
   is enough to fix that.

 * Make nsPresContext::UseOverlayScrollbars() thread-safe: We store the
   RDM pane stuff in the pres context.

The rest is pretty straight-forward.

Differential Revision: https://phabricator.services.mozilla.com/D168148