Revert "[heap] Separate map space pointers updating from main bulk"

This reverts commit 0f5d3ed1.

Reason for revert: Reland button relands right away...

Original change's description:
> [heap] Separate map space pointers updating from main bulk
> 
> Evacuating and compacting LayoutDescriptor is meta-circular, i.e., we
> need the descriptor to iterate the objects that are copied. Separate the
> phases to avoid requiring a publishing store for object payloads.
> 
> Bug: v8:6884
> Change-Id: Icc3bb80a938fbd9eec60c79d88ab8fdd9bda0840
> Reviewed-on: https://chromium-review.googlesource.com/700437
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48288}

TBR=ulan@chromium.org,mlippautz@chromium.org

Change-Id: Ie91ef6ea5a55f4e8165b3fa97b2c2042376998f1
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:6884
Reviewed-on: https://chromium-review.googlesource.com/701756Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48302}