We make it work on macOS by setting pointer-events: none + opacity: 0 rather
than visibility: hidden, and tweaking the caching setup to be Android-like.
Now that the scrollbars sheet is the same across platforms, move it to where
the rest of the UA sheets are. This way we guarantee that the RDM vs. Android
difference is less (just the ifdef at the top of the sheet).
Depends on D128084
Differential Revision: https://phabricator.services.mozilla.com/D128085
ad152d93ae