Some features (ex. CSSValuePool) want to keep a static per thread variable, but static Persistents cannot be used off the main-thread today.

We should introduce something like StaticPersistent that registers itself with the ThreadState and then gets cleared when the thread is shutting down. Then you could write:

DEFINE_THREAD_SAFE_STATIC_LOCAL(ThreadSpecific<StaticPersistent<MyObject>>, object, ())

and have a per thread static GC object.

ThreadState already keeps a set of PersistentNodes I think we can just do something similar to that?