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: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11480