Disable Mac-specific `backface-visibility` rules that cause Windows text
rendering (on newer Chromiums) to strop doing subpixel antialiasing (aka
ClearType) anywhere in the UI.

Fixes bug #9978 ([CEF 2171] Significant decrease in font quality on Windows).

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.
Merge pull request #10003 from adobe/pflynn/fix-cef-subpixel-aa

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