... this will avoid the need to pass AllocationPolicy to every method
that can allocate/deallocate and allows to make deallocation method
implementation stateful.

The latter will also allow implementing accounting of deallocated zone
memory.

Adding one more field is generally fine because usually these hashmap
objects are allocated on the stack or inside other rarely-allocated
long-lived objects.
The only exception is Scope class. The Scope objects are created very
often during parsing and each of them has a VariableMap field.
The Scope object size issue will be addressed in a follow-up CL.

Bug: v8:10572
Change-Id: I63fbd41246cf2e568c8ba80c213d3e9caffc2c87
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2284992Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68877}