Ensure that the type is always stored correctly.

R=titzer@chromium.org

Bug: v8:6600, chromium:791810
Change-Id: Id3a3c20b14f8730b9550c548dec49ac47121e691
Reviewed-on: https://chromium-review.googlesource.com/811188Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49924}