Shape also contains the Key type. Read it through there instead.

Bug: v8:6474
Also-By: cbruni@chromium.org
Change-Id: Ia47f6143c671c104901a714d4151c9fe9f525b5f
Reviewed-on: https://chromium-review.googlesource.com/529305Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45818}