Keep menubar from spazzing out in narrow screens

Summary:
If you resize your window to be very narrow, the menu bar spazzes out right now.
Prevent it from developing all sorts of weird internal linewrapping.

Test Plan:
Narrowed my browser window, header didn't spaz out.

Reviewed By: jungejason
Reviewers: aran, jungejason, tuomaspelkonen
CC: aran, jungejason
Differential Revision: 428