The field in question is a phantom link back into the GC'ed heap from
the native WebAssembly heap. It is one of the last pieces that makes the
compiled module specific to an Isolate. This is intended to finally make
code sharable between Isolates.

R=herhut@chromium.org
BUG=v8:7424

Change-Id: I75bcfabaf5149ca98a75d3ea3f19d8d8a8d96dce
Reviewed-on: https://chromium-review.googlesource.com/1113452Reviewed-by: Stephan Herhut <herhut@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54019}