This fixes a bug affecting module namespace objects, which are currently
implemented using native accessors.

Bug: v8:6681, v8:1569
Change-Id: I6a678652573a332c47315497d927c390d9da0926
Reviewed-on: https://chromium-review.googlesource.com/606027
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47238}