... which gets the zone from its ZoneAllocationPolicy instance.

This recovers memory regression caused by adding an AllocationPolicy
instance into TemplateHashMapImpl and therefore to VariableMap.

Bug: v8:10572
Change-Id: I7962b49e5f2669307e58b3ed7b1f29bab1c42cad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2298002Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68879}