The framework calls this when it is done with a particular layer. On the VM side, we can't dispose of all native resources at this point because something else might still need to hold on to the raster layer to rasterize a scene on the raster thread. On CanvasKit, it's slightly less obvious to me what we need to do - multi-threaded races aren't a concern, but we need to figure out when and whether to release resources on the native side.

/cc @yjbanov @hterkelsen