Fix window-exiting drags and drag-and-hold behaviors in JX.Scrollbar

Summary:
Fixes two issues:

• In Firefox, dragging outside the window and releasing the mouse button would miss the mouseup event. This would leave the bar dragging, even though the user had released the mouse button.
• In all browsers, dragging the handle and then holding your cursor in one place for more than a second would hide the handle. Instead, never hide the handle during a drag.

Test Plan:

• In Firefox, dragged handle right (outside of window) and released mouse button. Waved cursor over window; no more "sticky" scroll.
• In FF/Chrome/Safari, dragged handle and held cursor in same position for several seconds. No more handle hide.
• Waved cursor over window and made sure normal hiding still works.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11487