Summary:
Older versions of JSC (ios 11 and before) have a bug which I
believe the ProtectionQueue mechanism tickles:
https://bugs.webkit.org/show_bug.cgi?id=186827
This removes the ProtectionQueue and replaces it with an atomic flag
to avoid calling unprotect after VM shutdown.
This also fixes a race condition in shutdown.
Reviewed By: danzimm
Differential Revision: D12969953
fbshipit-source-id: fa3a14f3207be67a987ac3cf0fc1c9ce88837b0b