Consider removing controllerStyles and requiring explicitly adding stylesheets

[sorvell]

This feature was added initially as a styling polyfill stopgap. It's been used now in a few polymer-ui-elements. In the interest of simplicity, it may be better to remove the feature and to simply require stylesheets to be added where needed.

[sorvell]

Some examples include polymer-overlay, polymer-collapse, polymer-flex-layout. These elements provide behavior which can be applied to another target element. They want to control the styling of the target element and they do so now by modifying the set of css in the target's scope and setting selectors on the target.

[frankiefu]

We no longer use installControllerStyles in those elements.