The code and prototype_or_initial_map fields are marked as weak in the
torque definition of JSFunction. Given we don't have any annotations
that generate BodyDescriptors or the C++ class specifying weak there
didn't reflect in the actual C++ implementation.

In the C++ implementation we were treating those as strong pointers
and the rest of the code also expects them as strong pointers and
doesn't actually deal with cleared weak references.

Since JSFunction's header doesn't have any weak references we can just
fallback to JSObject::BodyDescriptor that iterates through all header
fields and the other JSObject fields as strong pointers. This is what
was happening because JSFunction didn't have a body descriptor and
we were falling back to JSObject::BodyDescriptor

So this is a clean up CL to avoid confusion. There shouldn't be
any change in behaviour.

So this CL basically:
1. Drops weak from the torque definition to match with implementation
2. Changes JSFunction::BodyDescriptor to JSObject::BodyDescriptor when
iterating over objects to be consistent with other JSObjects with
strong pointers.

Change-Id: I7c3e49f1759164a0f2517b3d5b8d0e5169b025ff
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2987827Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Mythri Alle <mythria@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75387}