Fix issue focusing on translate.ka.org perseus items.

Summary:
Perseus bridge tries to focus the item after rendering, but this.widgetIds was
not set on translated.ka.org at that time and so focus() was throwing a js
error.

Test Plan:
make all
See that there are no js errors on focus on:
http://translate.khanacademy.dev/preview/content/items?exercises=defining-similarity-through-angle-preserving-transformations&mp-r-id=38d4P_c=#x84260325bf2f749c

Reviewers: alpert

Reviewed By: alpert

Subscribers: jack

Differential Revision: http://phabricator.khanacademy.org/D11108