Fix bug #9978 (CEF 2171: Significant decrease in font quality on Windows).

Fix bug #9978 -- Disable Mac-specific `backface-visibility` styles that cause Windows text rendering to stop doing subpixel antialiasing (aka ClearType) anywhere in the UI, on newer Chromiums.

Note: there are a lot of `-webkit-font-smoothing: antialiased;` rules that seem like they might cause similar problems on Win, but actually that CSS property is specific to Apple text rendering, and does nothing on other platforms.