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: Ib3dcdb4559821d0ad9ca8cced37754e0e3c1d578
Reviewed-on: https://chromium-review.googlesource.com/943781
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51687}