Currently the concurrent marker iterates all fields in JSObjects up to
the instance size defined by the map. This can lead to a race when the
object transitions to unboxed double field.

Bug: chromium:774644
Change-Id: I01a69240869217127769bba9ff1c49dc5a81fa9c
Reviewed-on: https://chromium-review.googlesource.com/730717Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48817}