AFAIK, V8 already has 3 tiers: full-codegen (the basic non-optimizing JIT), crankshaft and turbofan. With ignition, they're replacing full-codegen with an interpreter.
It's strange that there's only one mention of crankshaft in the entire document, but turbofan is all over the place. Are they also planning to get rid of the former?
Spidermonkey is three tier and JSC is four.