It's the same as JSFunction::BodyDescriptor, so the special cases can be
removed.

This is a pre-step for refactoring BodyDescriptorWeak away.

BUG=v8:7308

Change-Id: Id65ce79f044960168d290fbbfdbc2a7aa854230e
Reviewed-on: https://chromium-review.googlesource.com/1226592
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55909}