Make the layer API suitable for both Flutter for mobile and for web

This is only here for tracking the progress on the unification of the layer API as part of https://github.com/flutter/flutter/projects/47

These fix is in https://github.com/flutter/flutter/pull/36402 that's being reviewed right now.