src: serialize both BaseObject slots #48996
Merged
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We previously only return startup data for the first slot for BaseObjects because we can already serialize all the necessary information in one go, but slots that do not get special startup data would be serialized verbatim which means that the pointer addresses are going to be part of the snapshot blob, resulting in indeterminism.
This patch updates the serialization routines and capture information for both of the two slots - the first slot with type information and memory management type (which we can use in the future for cppgc-managed objects) and the second slot with data about the object itself. This way the embeedder slots can be serialized in a reproducible manner in the snapshot.
Refs: nodejs/build#3043