This moves the link from a {WasmInstanceObject} to its corresponding
{WasmModuleObject} into the right place and also makes it strong. This
ensures that an instance always keeps the underlying module alive and
hence removes the situation of an "orphaned instance".

R=clemensh@chromium.org

Change-Id: Id59f6a49740af8ef0248679c3d2c696bb9776944
Reviewed-on: https://chromium-review.googlesource.com/1041691Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52942}