For namespace objects, [[GetOwnProperty]] on an uninitialized property
throws a ReferenceError. This was not implemented everywhere. This CL
fixes all such issues I'm aware of.

Bug: v8:7470
Cq-Include-Trybots: luci.v8.try:v8_linux_noi18n_rel_ng
Change-Id: I5f024450005c4f4dcb3f41c844ef055f67a9a869
Reviewed-on: https://chromium-review.googlesource.com/937341Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51638}