Unification: now BodyDescriptor deals with all weakness types.

This doesn't replace the weak list pointers with in-place weak references, since
it would cause extra work: we anyway recreate the lists after GC, so we
shouldn't track them at all during GC.

BUG=v8:7308

Change-Id: Ifb2f573d3e7ee311136b59e185cc659487c9cab3
Reviewed-on: https://chromium-review.googlesource.com/1229894Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56030}