[css-layout-api] Updates before F2F

Hi all,

I've created an explainer
<https://github.com/w3c/css-houdini-drafts/blob/master/css-layout-api/EXPLAINER.md>
on github which contains an overview of the API proposal so far. The spec
<https://drafts.css-houdini.org/css-layout-api/> has also updated which is
much more complete than last time.

Explicit things I'd like feedback on are:
 - How to handle min/max content contributions. I think this is the last
major thing to nail down. We've talked about this previously about how we
can do this either with an additional author defined method, or via the
existing layout method; but I'd like to gain consensus here.
 - Any nits on naming and structure of the API in general. E.g. how this
API handles scrolling, the utility functions, etc.

Thanks,
Ian