This reverts commit a0c57368.

Reason for revert: Speculative revert due to failures with custom
snapshot:
https://build.chromium.org/p/client.v8/builders/V8%20Linux64%20GC%20Stress%20-%20custom%20snapshot/builds/19061

Local bisect also points to this change:
http://shortn/_IhVxU2FKLu

Original change's description:
> [wasm] Maintain link from Instance to Module.
> 
> 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/1041691
> Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
> Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#52942}

TBR=mstarzinger@chromium.org,clemensh@chromium.org

Change-Id: I1830e6ce14314f06f918a0c428182bfd68354ad9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/1041968Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52949}