Forward reference resolution writes didn't have a write barrier,
which means the slot wouldn't be recorded if there was an active
slot recording marker running.

Now use the same SlotAccessor interface as the other deserializer
writes, to make sure that the correct write barrier is called.

As a drive-by, clean up SlotAccessorForHeapObject into two static
constructors, to differentiate between access by slot index and
offset.

Fixed: v8:11065
Bug: v8:10460
Change-Id: I5b3a3d94057763324d6e1727d96b65c73ba5d7b4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2504263
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70839}