Previously ShouldHaveBeenSerialized() would return false for
kPossiblyBackgroundSerializedHeapObject objects which prevented
checks for whether the correct serialization had been done before
accessing Map::prototype() for these ObjectRefs.

BUG=chromium:1168435,v8:7790,v8:9684

Change-Id: I31b4cf7c7ce67ba1c46aea1451172b279d215508
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2640479
Auto-Submit: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72215}