Make the row on the chart clickable in order to also zoom in the pie-chart.
Make the row on the chart clickable in order to also zoom in the pie-chart.

Test Plan:
in webapp: arc patch D15880
http://es.khanacademy.dev/translations
Click on the row, and see the pie chart update

Reviewers: emily, csilvers

Reviewed By: csilvers

Subscribers: csilvers

Differential Revision: https://phabricator.khanacademy.org/D15899

Khan/d3-hierarchical-pie