Almost all callers of BodyDescriptor already have the map of the object
and should pass it to IterateBody and IsValidSlot functions.

This removes redundant load and makes the function consistent with the
SizeOf function.

Change-Id: Ie47a9bb05af23fbf0576dff99f2ec69625e057fc
Reviewed-on: https://chromium-review.googlesource.com/979436
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52218}