This CL fixes a bug where a fast-path was used on non-extensible
objects.

R=jgruber@chromium.org

Bug: chromium:865264,chromium:865285
Change-Id: Ie14c95b383a65576799c71576a5c0f9f8e1c29ca
Reviewed-on: https://chromium-review.googlesource.com/1142766Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#54539}