The main changes of this CL are:

It should no longer be assumed that an empty ArrayBuffer has a nullptr
backing store. This is in preparation for the move to caged pointers,
which cannot represent nullptr, and will instead likely provide a
EmptyBackingStore constant pointing inside the virtual memory cage. For
that reason, a new JSArrayBuffer::IsEmpty() helper is introduced, which
should be used instead of checking against nullptr.

CodeStubAssembler::GetTypedArrayBuffer now checks for on-heap
TypedArrays instead of comparing the backing store pointer to nullptr.
This is consistent with the implementation in JSTypedArray::GetBuffer.

v8::ArrayBufferView::CopyContents now uses JSTypedArray::DataPtr instead
of relying on nullptr backing stores to handle on-heap TypedArrays.

The serializer and deserializer now check for IsEmpty() and use the
kEmptyBackingStoreRefSentinel value to serialize empty backing stores.

Empty ArrayBuffers allocated for on-heap TypedArrays now have a
byte_length of zero. This allows removing the allocation_length() (and
allocation_buffer()) methods, which were only (incorrectly, as they
don't account for GSABs) used for memory measurements.

Bug: chromium:1218005
Change-Id: Ib889ccf855f68525f7a614f3963e46ea56865fa3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3297709Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78069}