Allowing GetModuleNamespace on a not-yet-evaluated module does not make a
lot of sense because accessing the namespace object before evaluation
can lead to surprising behavior.

R=adamk@chromium.org

Bug: v8:1569
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: I3e3feb344f6399bf92b3dabc97c571a61b38bd41
Reviewed-on: https://chromium-review.googlesource.com/613268Reviewed-by: Adam Klein <adamk@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47365}