To accomplish this, I made the following changes:

1) Material is now in charge of drawing the material shadows.
2) In order to mix in the style for the shadow, Element now takes a list of
   Styles instead of a single style.
3) Update all clients of Element#style to understand that we now have a list.
4) Update components that drawer shadows to have Material do that work instead.
   a) One exception: FloatingActionButton draws its own shadow because of its
      crazy clip requirements. We'll probably want to find a better way for
      FloatingActionButton to clip in the future.

I've also added a widgets-fn example to demo the fn material widgets.

This CL introduces a bug into Drawer whereby you can get ink splashes
everywhere in the drawer. In the future, we'll need to separate out the
different material aspects to get non-splashable materials.

R=rafaelw@chromium.org

Review URL: https://codereview.chromium.org/1003553002