Add font-size back to textareas.

Summary:
By default, users who enable monospaced textareas were getting 10px font sizes.
This is worse than the 12px non-monospace users saw which is what D4377 fixed.
This diff adds the font-size line back, but at the correct value of 13px instead
of the former 12.

Test Plan: Looked at a textarea in monospace and no longer saw 10px font.

Reviewers: chad

Reviewed By: chad

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D4399