It must be possible to determine an object's size on the heap without
relying on the presence of any other objects. Specifically, if an
object and its WasmTypeInfo die at the same time, they can be swept
in any order, and the sweeper may need to know their sizes.
This patch solves the problem by repurposing two bytes in the Map,
where WasmStructs can store their instance size, and WasmArrays can
store their element size (which can be used to compute their size).

Fixed: chromium:1240670
Change-Id: Ib960fd0a409936aff1aef4daafed4c38b8497880
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3106649
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76391}