A new Snapshot-Content-Location header is being added:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/GkAQbgmElK8/zx8KPu8WCgAJ

For MHTML in general, if we want it to be possible to save MHTML files in one browser and share them to another, then we would need some way of writing shared tests for at least parsing and serialization, probably in web-platform-tests.

This might require WebDriver extensions to save a page as MHTML in the first place, as there's no web-exposed API for this.