Let a `CollectionSupportingView` invalidate its own layout

It would be great to allow a `CollectionSupportingView` to be able to invalidate it's own layout in response to some view state that changes it's bounds.

Eg. a input text field that grows in height as the user types.

See https://github.com/dropbox/pilot/pull/102 for a naive implementation.

That implementation needs 2 things to be generally useful in pilot:
1. It should only invalidate the layout of the single view (not the whole CV)
1. It should have a more robust way of getting a reference to the collectionView.