This will allow the owner of a button bar to react to changes in the button bar's intrinsic size.
We need to add this delegate because I'm not aware of an equivalent UIKit mechanism that works with views that aren't using auto layout (the MDCNavigationBar uses manual layout).
In a follow-up change, MDCNavigationBar will implement this delegate and update the layout of its left/right button bars as a result.
Part of https://github.com/material-components/material-components-ios/issues/1717
