Allow scrollbars to be dragged outside the window on supporting browsers

Summary: In Safari, Firefox and Chrome drags outside the window will work if we do this. Safari didn't work before, not sure about the other two.

Test Plan: Clicked the scroll handle, then dragged my mouse to the right (outside the window) and down. Page scrolled in Safari, Firefox, and Chrome.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11484