The layout descriptor helper computes the object header size using
map->instance_size() and map->GetInObjectProperties().

It races with finalization of slack tracking, which changes both
the instance size and the in-object properties count.

This patch replaces the in-object properties count byte in the map
with the byte that stores the start offset of in-object properties.

The new byte can be used in the layout descriptor to compute the
object header size and it is immutable.

This patch also renames InstanceSize to InstanceSizeInWords where
the instance size is represented in words.

Bug: chromium:786069, chromium:694255
Change-Id: I4b48c6944d3fe8a950bd7b0ba43d75216b177a78
Reviewed-on: https://chromium-review.googlesource.com/776720
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49461}