When you make the file tree huge, scroll to the right, and then toggle it, stop it from growing

Summary: Depends on D19346. Ref PHI568. I love Javascript.

Test Plan:

• Viewed a revision.
• Dragged file tree view really wide.
• Scrolled document to the right.
• Toggled file tree off and on by pressing "f" twice.
  • Before patch: file tree grew wider and wider after it was toggled.
  • After patch: file tree stayed the same size after it was toggled.
• Dragged to various widths and reloaded to make sure the "sticky across reloads" behavior still works.
• Scrolled right, dragged the tree a bit, then reloaded and didn't see it flip out.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19347