The NativeModule that was serialized in another isolate might be
temporarily kept alive by background threads. By keeping a weak pointer
to the module, we can wait for it to really die.
This happens very rarely, and the module will die pretty quickly, so
busy-waiting is fine in this case.

R=thibaudm@chromium.org

Bug: v8:10148
Change-Id: I8c4645acfccd04a820ef3f694cad3eb15e75acb6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2016585Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65942}