Deserialization bypasses the heap allocators, bumping pointers into the
spaces directly, instead.  So the deserializer is tightly coupled to the
implementation of the existing collector.

Here I've added an interface to heap.h for this purpose.  This CL
leaves things as-is unless the TPH is enabled, in which case the new
interface is used.

Future work: use the heap.h interface in all cases.

Bug: v8:9533

Change-Id: I3b1cc81870b347fbfb509ddb4031bd3781710240
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2019482
Commit-Queue: Steve Blackburn <steveblackburn@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65992}