Two threads might get the same PersistentNode because the
BasicCrossThreadPersistent ctor wasn't taking a lock. Then if one thread
frees the node and the other initalizes it or updates its owner, we get
some random object in our free list of PersistentNodes.

I debug a crash in Assign(Unsafe) and Clear where the PersistentNode
seemed to be allocated on stack. Empirically, adding this guard resolved
it. I can't confirm in the code that the scenario above is what was
happening.

Drive-by: adding a few DCHECKs.

Bug: chromium:1056170
Change-Id: I37d8ed5bb942a124c98d7524b7f04fe8ccb2aefd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2718144
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73023}