Add reactive backgroundColor UIView property.
Add reactive backgroundColor UIView property.

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, appsforartists

Reviewed By: O2 Material Motion, #material_motion, appsforartists

Tags: #material_motion

Differential Revision: http://codereview.cc/D2694

material-motion/material-motion-swift,material-motion/material-motion-swift