Currently it was only used for modules, so repurposed it to be a weak pointer to
JSModuleNamespace.

BUG=v8:7308

Change-Id: I4ef522fafebd37624c309081d7432501c2c69b7a
Reviewed-on: https://chromium-review.googlesource.com/1163704Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55013}