BodyDescriptorWeak is not needed for all classes. For the classes it's needed
it's referred to explicitly (Foo::BodyDescriptorWeak::IterateBody()).

BUG=v8:7308

Change-Id: If8591929cd588575e88f3d6f116ec2cac4941e8f
Reviewed-on: https://chromium-review.googlesource.com/1226649Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55941}