This allows an embedder to check if a Value is a module namespace object.

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Idffceff451dd5f5c6a53d4cb3ce02c1c2c5b653c
Reviewed-on: https://chromium-review.googlesource.com/1011762Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52597}