Fix an issue with dragging the scrollbar handle from a noninitial position

Summary:
See <rPc40bc0c8bf75#4050>. Repro steps:

• Scroll partway down the page.
• Click and drag the scroll handle.

Prior to this diff, the handle incorrectly jumps back to the top of the page. This is because we didn't store the handle's original position. (In testing, I always dragged from near the top of the page, and I don't normally drag scrollbars, so I didn't notice this.)

Test Plan: Clicking and dragging a partially scrolled handle now works correctly.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11483