In particular: remove PossiblyBackgroundSerializedHeapObject in favor of
either SerializedHeapObject or BackgroundSerializedHeapObject, depending
on the (existing) argument to the ObjectRef constructor.

Bug: v8:7790
Change-Id: I0a67872c377107799233742cc7774bea62811d0f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2681946
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72591}