Instead of storing a weak pointer per isolate, store exactly one weak
pointer to the native module per engine.
This is a small preparation for switching to the jobs API.

R=ahaas@chromium.org

Bug: chromium:1101340
Change-Id: I5f6590421c890998aa95a0a3b34596f59f2f2690
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2297471Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68858}