Store it as FixedArray instead of Object. Use the empty array as
uninitialized value instead of undefined.

R=titzer@chromium.org

Bug: v8:8015
Change-Id: I53275dc5fd2c3a0db1e021ee4da0fc0694e2053e
Reviewed-on: https://chromium-review.googlesource.com/1224372Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55867}