[TurboFan] Access heap based on data mode instead of broker mode

ObjectRefs either use the serialized data or access the heap depending
on whether broker mode is disabled or not. This cl changes it to use
the mode of the data and accesses heap for UnserializedHeapObject. This
allows us to access heap for some of the objects (ex: readonly object)
instead of serializing them.

This change shouldn't change any behaviour. In a followup cl, we will
mark readonly objects as UnserializedHeapObjects even when broker mode
is serialized.

Bug: v8:9684
Change-Id: I8775e60ed480957b485c39df254827dd69c655a8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1937148Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65206}