This must throw for uninitialized properties.

R=adamk@chromium.org

Bug: v8:1569, v8:5487
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng;master.tryserver.v8:v8_linux_noi18n_rel_ng
Change-Id: I8beb8bf6a197870eb5c038102ab474dd12f6b6eb
Reviewed-on: https://chromium-review.googlesource.com/582013
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46870}