This makes sure that the object can be identified by a unique instance
type and hence is not accidentally confused with other FixedArrays on
the heap.

R=clemensh@chromium.org
BUG=v8:7509

Change-Id: I20521cdcabbbddecd89ca8cd4bb203a47e1db0cd
Reviewed-on: https://chromium-review.googlesource.com/946253Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51730}