I think we talked about this a long time ago when page_state.mojom was in the works. I guess we'd implement this as a PRESUBMIT check, and an IDL parser would verify that the before and the after are written in a backwards compatible way?

I'm not sure if we want those for both interfaces and structs, but probably? Enums and unions as well.

To start the naming bikeshed:
StableForSerialization?
Serializable?
Serialized?
BackAndForwardsCompatible?