Don't show bar for measurer

Summary:
This implements a kind of hacky fix for widgets which shouldn't be
highlighted.

Test Plan:
* Open test.html
* Add a non-measurer widget
* see the highlight
* Add a measurer
* See no highlight

Reviewers: jack, alex

Reviewed By: alex

Subscribers: alpert

Differential Revision: http://phabricator.khanacademy.org/D7571