Body descriptors are used by marking and scavenging visitors.

Change-Id: I6912bb5b924755db5750f0a3b1e4909bff5375c7
Reviewed-on: https://chromium-review.googlesource.com/732978Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48901}