R1:512b17361845

Don't activate JX.Scrollbar if we don't need to

Summary:
See discussion on rPc40bc0c8bf75. Fixes a couple of glitchy things:

- Things were generally not nice on iOS.
- On OSX, with no mouse, the OS scrollbar and our fake scrollbar would both draw.
- Bar z-index was not set quite correctly.

Specifically, check if we need these bars. If we don't, just exit immediately and use the OS bars.

Test Plan:
- Tested Safari, Firefox, Chrome with and without a mouse.
- Tested iOS Simualtor.

Reviewers…