Fix font size in textarea

Summary: font-family: inherit doesn't inherit the font size resulting in using default monospace font size for inherited font family which results in too big font.

Test Plan: Wrote a comment, wrote an inline comment.

Reviewers: nh, chad, epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3997