This was done via {managed_native_module()->get()}. Add a simple getter
for that.

R=ahaas@chromium.org

Bug: v8:8562
Change-Id: I8e461a8e16b618abdb772098fad3a6b721d54902
Reviewed-on: https://chromium-review.googlesource.com/c/1371564Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58156}