The motivation for the new field is to provide race-free way to
iterate used in-object properties of a JSObject in concurrent marker.

This CL keeps the new field in sync with the unused_property_fields
and subsequent CL will remove unused_property_fields.

Bug: chromium:774644
Change-Id: I0971f079094d58d3a57415834c43c09427dacc77
Reviewed-on: https://chromium-review.googlesource.com/726639
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48795}