miguely 644139fb99 Add a BackLayerLayout and BackLayerSiblingBehavior + tests.
These are the foundation for the back layer functionality. The behavior
does the heavy lifting to measure the adjacent content view.

As of now this only lays out the views correctly in the hidden state, it doesn't support exposing the back layer at all, yet. Full of TODOs, this is just a skeleton.

PiperOrigin-RevId: 156334640
2017-05-24 16:33:21 -07:00
..