When tiering down (or up), we first get a list of all native modules
(under a lock), then tier them down/up without holding the lock. Since
we don't hold (shared) ownership of the native module, it could die
in-between.
This CL fixes this by keeping weak pointers to the native modules, and
re-gaining a shared pointer before putting the module in the list of
modules to be tiered down/up.

R=thibaudm@chromium.org

Bug: v8:10588
Change-Id: I2891c3729f42f26d4026f3e2448e124863b95122
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_isolates_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2228515
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68145}