It will allow us to use knowledge about the type of field during value
decompression upon field read.

Use the new class for HeapObject::MapField.

Bug: v8:9353
Change-Id: I1368426ec2e25fcec3af8d5cccd7a78d80423e72
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1658150
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62184}